Cookbook

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This guide is a practical path from a first factory to the patterns that tend to appear in a real test suite. It uses the same account-and-blog examples as the rest of the documentation so that you can move between the README, this cookbook, and the FactoryMan API reference without learning a new example domain each time.

The examples assume Ecto schemas such as MyApp.Accounts.User, MyApp.Blog.Author, MyApp.Blog.Post, MyApp.Blog.Tag, and MyApp.Blog.Comment. Replace those modules and fields with the ones in your application.

Two conventions appear throughout the guide:

  • Struct factories receive a map, define defaults in base_params, and finish with Map.merge(base_params, params) so caller values win.
  • Helpers are called with the module name, such as FactoryMan.sequence(...) and FactoryMan.assoc(...). Only deffactory and defvariant are imported by use FactoryMan.

Start with one useful factory

A factory is an ordinary module that uses FactoryMan. Configure your repo once, alias the schema, and return a map of fields from the factory body:

defmodule MyApp.Factory do
  use FactoryMan, repo: MyApp.Repo

  alias MyApp.Accounts.User

  deffactory user(params \\ %{}), struct: User do
    base_params = %{
      username: FactoryMan.sequence("user"),
      email: FactoryMan.sequence(:email, fn n -> "user#{n}@example.com" end),
      role: "member",
      joined_at: fn -> DateTime.utc_now() end
    }

    Map.merge(base_params, params)
  end
end

The defaults make the factory useful without arguments, while the final merge keeps individual tests in control:

test "shows the member dashboard" do
  user = MyApp.Factory.build_user_struct(%{username: "alice"})

  assert user.username == "alice"
  assert user.role == "member"
  assert user.id == nil
end

FactoryMan builds the struct for you. Do not return %User{} from a normal struct factory; use body: :struct only when you deliberately need direct control over struct construction.

Choose the result that matches the test

A struct factory generates several related functions. Use the function that matches the boundary you are testing instead of building one representation and converting it by hand.

The test needsUse
An in-memory recordbuild_user_struct/0,1
An atom-keyed input mapbuild_user_params/0,1
A string-keyed request mapbuild_user_string_params/0,1
Several independently built valuesThe matching *_list function
A row that already exists in the databaseinsert_user/0,1,2

Use a struct when the code under test takes a record and no database row is needed:

test "formats a display name" do
  user = MyApp.Factory.build_user_struct(%{username: "alice"})

  assert MyApp.Accounts.display_name(user) == "@alice"
end

Use params when exercising a changeset or context boundary:

test "accepts valid registration attributes" do
  attrs = MyApp.Factory.build_user_params(%{username: "alice"})

  assert %Ecto.Changeset{valid?: true} =
           MyApp.Accounts.User.changeset(%MyApp.Accounts.User{}, attrs)
end

Use string params when the real caller supplies string keys, as a controller or API client would:

test "creates a user through the API", %{conn: conn} do
  params = MyApp.Factory.build_user_string_params(%{username: "alice"})

  conn = post(conn, ~p"/api/users", %{"user" => params})

  assert %{"username" => "alice"} = json_response(conn, 201)
end

Params builders first build the struct and then remove Ecto metadata and persistence-only fields. That is why they are preferable to calling Map.from_struct/1 yourself.

Use an insert when persistence is part of the behavior under test:

test "loads a user by username" do
  user = MyApp.Factory.insert_user(%{username: "alice"})

  assert MyApp.Repo.get_by!(MyApp.Accounts.User, username: "alice").id == user.id
end

Build several independent values

List builders invoke the factory once per item. Sequences, lazy values, hooks, and association builders therefore run independently for every result:

users = MyApp.Factory.build_user_struct_list(3, %{role: "moderator"})

assert length(users) == 3
assert Enum.all?(users, &(&1.role == "moderator"))
assert users |> Enum.map(& &1.username) |> Enum.uniq() |> length() == 3

The same pattern applies to params and inserts:

attrs_list = MyApp.Factory.build_user_params_list(3)
inserted_users = MyApp.Factory.insert_user_list(3, %{role: "moderator"})

If you modify an already-built struct before inserting it, keep the factory's insert hooks by using insert_user_struct/1,2 rather than calling the repo directly:

user = MyApp.Factory.build_user_struct()
edited_user = %{user | username: "edited"}

MyApp.Factory.insert_user_struct(edited_user)

Make defaults realistic

Useful defaults should resemble valid application data, avoid accidental collisions, and defer work that may be overridden by the caller.

Generate unique values with sequences

The simplest sequence prefixes a counter with a string:

FactoryMan.sequence("user")
# "user0", then "user1", then "user2", ...

A formatter gives the counter an application-specific shape:

FactoryMan.sequence(:email, fn n -> "user#{n}@example.com" end)
FactoryMan.sequence(:order, fn n -> "ORD-#{n}" end, start_at: 1000)

A list formatter cycles through a small set of values:

FactoryMan.sequence(:role, ["admin", "member", "guest"])

Cycling values are useful for varied data, but they are not unique. Use a formatter when a database constraint requires uniqueness.

Reset sequence state when a test asserts exact generated values:

setup do
  FactoryMan.Sequence.reset()
  :ok
end

Sequence state is shared, so tests that reset and assert exact sequence positions should not race with other tests using the same sequence names.

Compute values lazily

Function values are evaluated when the factory builds. A zero-arity function computes a fresh value, and a one-arity function receives the containing map:

deffactory user(params \\ %{}), struct: User do
  base_params = %{
    username: FactoryMan.sequence("user"),
    role: "member",
    joined_at: fn -> DateTime.utc_now() end,
    display_name: fn user -> "#{user.username} (#{user.role})" end
  }

  Map.merge(base_params, params)
end

This is useful in a test that cares about a value derived from another caller-controlled field:

user = MyApp.Factory.build_user_struct(%{username: "alice", role: "admin"})

assert user.display_name == "alice (admin)"

Lazy values are resolved in two passes. The zero-arity functions run first, then the one-arity functions receive the result. A one-arity function can therefore read a plain field such as username, or a zero-arity field such as joined_at, but not another one-arity field, which is still a function reference when it runs:

base_params = %{
  role: "member",
  joined_at: fn -> DateTime.utc_now() end,
  # Reads a plain field and a resolved zero-arity field
  summary: fn user -> "#{user.role} since #{user.joined_at.year}" end
}

Because the caller's params are merged before any of this happens, an override flows into the derived value as well:

user = MyApp.Factory.build_user_struct(%{role: "admin"})

assert user.summary == "admin since #{user.joined_at.year}"

Lazy defaults also avoid work when a caller supplies an override. An eager default is computed before the merge, even when the caller's value replaces it:

base_params = %{
  # Advances the sequence only when the caller does not supply a username
  username: fn -> FactoryMan.sequence("user") end
}

Map.merge(base_params, params)

For associations, declare a builder with assocs: instead (see Build related data). The builder is the default, and it runs only when the caller does not supply the key.

Name recurring scenarios with variants

A variant is a small preprocessor for a base factory. Use one when tests repeatedly need the same kind of record:

defvariant admin(params \\ %{}), for: :user do
  base_params = %{role: "admin"}

  Map.merge(base_params, params)
end

defvariant guest(params \\ %{}), for: :user do
  base_params = %{role: "guest", username: "guest"}

  Map.merge(base_params, params)
end

FactoryMan combines the variant and base names:

admin = MyApp.Factory.build_admin_user_struct()
guest = MyApp.Factory.insert_guest_user()

assert admin.role == "admin"
assert guest.role == "guest"

A variant runs before the base factory. With the canonical merge order, callers can still override the preset:

user = MyApp.Factory.build_admin_user_struct(%{role: "owner"})
assert user.role == "owner"

That behavior is useful for defaults. If the name promises an invariant that callers must not contradict, force the value or validate it explicitly (see Build presets that keep their promises).

Combine variants for one test

Pass variants: to any generated function to apply several variants in one build:

MyApp.Factory.build_user_struct(%{username: "alice"}, variants: [:admin, :confirmed])
MyApp.Factory.insert_user(%{}, variants: [:admin, :confirmed])

List variants by the name in their defvariant (:admin, not :admin_user). The caller's params win, and a later variant wins over an earlier one:

user = MyApp.Factory.build_user_struct(%{}, variants: [:admin, :guest])
assert user.role == "guest"

insert_* takes one option list: FactoryMan uses :variants, and every other option goes to Repo.insert!/2 unchanged, as in insert_user(%{}, variants: [:admin], returning: true).

Build on another variant

When a variant always includes another, declare it with extends:. The variant wins over the variants it extends:

defvariant senior(params \\ %{}), for: :user, extends: [:admin] do
  Map.merge(%{display_name: "Senior administrator"}, params)
end

user = MyApp.Factory.build_senior_user_struct()
assert user.role == "admin"

A variant that only extends others names a combination that many tests use:

defvariant confirmed_admin, for: :user, extends: [:admin, :confirmed]

MyApp.Factory.build_confirmed_admin_user_struct()

for: always names the base factory, and each extended variant must be defined earlier in the same module.

Choose defaults or forced values

A variant's defaults can be overridden by the caller; its forced values cannot. Use defaults for convenience presets, and forced values for presets whose name is a promise. A variant that only sets values declares them:

# Defaults: the caller can override the role
defvariant admin, for: :user, defaults: %{role: "admin"}

# Forced: a banned user is always banned, whatever the caller passes
defvariant banned, for: :user, force: %{banned: true}

defaults: and force: are evaluated on every build. Wrap a value in a lazy function when it should only be computed when the caller does not supply the key:

defvariant confirmed, for: :user, defaults: %{confirmed_at: fn -> DateTime.utc_now() end}

When the values are computed from the params, write a body instead. Which side of Map.merge/2 the body puts params on decides who wins:

defvariant renamed(params \\ %{}), for: :user do
  Map.merge(%{display_name: String.upcase(params[:username] || "user")}, params)
end

Add :factory_man to import_deps in .formatter.exs, so mix format keeps a variant without a body free of parentheses.

In a variants: list, a variant that forces a value wins over the caller and over every variant after it in the list.

Use as: when the combined name would be awkward:

defvariant moderator(params \\ %{}), for: :user, as: :mod do
  Map.merge(%{role: "moderator"}, params)
end

MyApp.Factory.build_mod_struct()

Ecto relationships are where factory setup can become noisy. Declare association builders with assocs:, and be explicit about whether the test needs an in-memory graph or rows that already exist in the database.

Declare association builders

Suppose account and blog factories live in separate modules:

defmodule MyApp.Factory.Accounts do
  use FactoryMan, extends: MyApp.Factory

  alias MyApp.Accounts.User

  deffactory user(params \\ %{}), struct: User do
    base_params = %{
      username: FactoryMan.sequence("user"),
      email: FactoryMan.sequence(:email, fn n -> "user#{n}@example.com" end)
    }

    Map.merge(base_params, params)
  end
end

defmodule MyApp.Factory.Blog do
  use FactoryMan, extends: MyApp.Factory

  alias MyApp.Blog.{Post, Tag}
  alias MyApp.Factory.Accounts

  deffactory tag(params \\ %{}), struct: Tag do
    base_params = %{name: FactoryMan.sequence("tag")}

    Map.merge(base_params, params)
  end

  deffactory post(params \\ %{}),
    struct: Post,
    assocs: [author: &Accounts.build_user_struct/1, tags: &build_tag_struct/1] do
    base_params = %{
      title: "#{params.author.username}'s first post",
      content: "A post written for a test"
    }

    Map.merge(base_params, params)
  end
end

Every declared key is resolved before the body runs, so the body always receives it resolved (here, params.author is a %User{}).

The builder is the default. An absent key is built with %{} (a singular association) or resolves to [] (a plural one). Don't also put a default for a declared key in base_params; it is never used. Use default: to change what an absent key means:

assocs: [
  # No editor unless the caller supplies one
  editor: {&Accounts.build_user_struct/1, default: nil},
  # A post comes with two tags
  tags: {&build_tag_struct/1, default: [%{name: "elixir"}, %{}]}
]

Callers can supply nested params, existing structs, or nil. A plural association may mix existing structs and params maps:

test "builds a post from nested input" do
  user = MyApp.Factory.Accounts.build_user_struct(%{username: "existing"})
  tag = MyApp.Factory.Blog.build_tag_struct(%{name: "existing-tag"})

  post = MyApp.Factory.Blog.build_post_struct(%{author: user, tags: [tag, %{name: "new-tag"}]})

  assert post.author === user
  assert [^tag, %Tag{name: "new-tag"}] = post.tags
end

The rules for each kind of caller value are in the FactoryMan module documentation and the cheat sheet.

Require an association

An explicit nil normally resolves to nil, because "no associated value" is often a real state. When it never is, mark the key required: true:

deffactory post(params \\ %{}),
  struct: Post,
  assocs: [author: {&Accounts.build_user_struct/1, required: true}] do
  Map.merge(%{title: "#{params.author.username}'s first post"}, params)
end
MyApp.Factory.Blog.build_post_struct(%{author: nil})
# ** (ArgumentError) association :author in factory :post in MyApp.Factory.Blog is required, got nil

required: true means the key resolves to a value, not that the caller must supply it: an absent key still builds. A builder that returns nil for a required key also raises. Builders declared below a required key can rely on it being set.

It applies to singular associations only (a plural association already raises on nil, and required: is not a check that the list is non-empty), and it can't be combined with default: nil. It is checked when the key resolves, so a body that puts nil back afterwards is not caught. A variant can add required: true to a key, but can't relax a base factory's.

Chain associations

Keys resolve top to bottom. A 2-arity builder also receives the factory params, with every key declared above it already resolved:

deffactory comment(params \\ %{}),
  struct: Comment,
  assocs: [
    author: &Accounts.build_user_struct/1,
    post: fn params, factory_params ->
      # The comment's post defaults to one written by the same author
      build_post_struct(Map.put_new(params, :author, factory_params.author))
    end
  ] do
  Map.merge(%{body: "Nice post"}, params)
end

In factory_params, the current key and every key declared below it are hidden until they resolve. Undeclared keys are visible as the caller passed them, which is how a builder can read an input that is not a declarable association. The body's base_params have not been computed yet, so a builder that needs one of those values computes it itself. A caller-supplied struct is reused without calling the builder. An earlier key may be nil when the caller passed nil, so write builders to handle that.

Build associations differently in a variant

defvariant accepts assocs: too. The variant resolves its keys before its body, and the base factory reuses the resulting structs, since supplied structs are never rebuilt:

defvariant guest(params \\ %{}),
  for: :post,
  assocs: [author: &MyApp.Factory.build_guest_user_struct/1] do
  Map.merge(%{title: "A guest post"}, params)
end

Share a declaration

assocs: takes any expression that returns a keyword list, so factories can share one:

deffactory post(params \\ %{}), struct: Post, assocs: authored() do
  Map.merge(%{title: "A post"}, params)
end

deffactory draft(params \\ %{}), struct: Post, assocs: authored() do
  Map.merge(%{title: "A draft", draft: true}, params)
end

def authored, do: [author: &Accounts.build_user_struct/1]

The expression is evaluated at build time, like the body, but the factory's params variable is not in scope in it. It must be written directly in the deffactory or defvariant call, not in options held in a module attribute.

Keep default: values literal

assocs: is evaluated on every build, so a default: value is too, whether or not the caller supplies the key. Any side effect inside one happens every time: an insert, a query, or a sequence, which advances even when the default goes unused. Keep defaults to literals, and put build logic in the builder, which runs only when it is needed.

A record at an association position inside default: (e.g. default: %{mentor: %User{}}) is rejected, because it has already been built. That check covers only records at association positions: %{author_id: MyApp.Factory.Accounts.insert_user().id} passes it and still inserts a user on every build.

Resolve associations imperatively

FactoryMan.assoc/3,4 and FactoryMan.assoc_list/3,4 apply the same rules to one key of a params map, without required: or the schema check. Use them where assocs: does not fit: a plain helper function, an association resolved from a value computed in the body, or inside a builder. Their only option is default:, with the same meaning as in assocs::

def insert_post_for(params) do
  author = FactoryMan.assoc(params, :author, &MyApp.Factory.Accounts.insert_user/1)

  MyApp.Factory.Blog.insert_post(Map.put(params, :author, author))
end

To give an absent key extra params, close over them:

FactoryMan.assoc(params, :author, &build_user_struct(Map.merge(%{role: "writer"}, &1)))

The helpers return the resolved value and do not modify the params map. In a body that ends with Map.merge(base_params, params), the merge would put the caller's raw value back over the resolved one, so handle the key before merging.

When an association is only an input used to derive other fields, and should not be stored on the struct, drop it:

deffactory author(params \\ %{}), struct: Author do
  # Only the user's row and name are needed, not the association itself
  user = FactoryMan.assoc(params, :user, &Accounts.insert_user/1)

  base_params = %{name: user.username, user_id: user.id}

  Map.merge(base_params, Map.drop(params, [:user]))
end

When the body changes a resolved association, put the changed value back:

deffactory post(params \\ %{}), struct: Post, assocs: [author: &Accounts.build_user_struct/1] do
  # The post's author is always marked as a writer
  author = %{params.author | role: "writer"}

  Map.merge(%{title: "A post"}, Map.put(params, :author, author))
end

Avoid recursive defaults

A self-referential association builds forever when its key is absent: building a user builds its mentor, which builds its mentor, and so on. So does a mutually recursive pair whose defaults build each other, such as a user declared with a non-empty default: for its posts, where each post builds its author. (An absent plural key resolves to [] unless default: says otherwise, so a list only loops when its default builds something.) FactoryMan raises instead:

** (ArgumentError) association :mentor in factory :user in MyApp.Factory.Accounts is
self-referential: building it by default builds :user again, which recurses forever.
Build path: :user → :mentor → :user → :mentor
Declare it as {builder, default: nil}, or pass a value (nil, a struct, or params).
If the recursion is meant to stop on its own, supply the key at each level instead.

Declare such a key with default: nil (default: [] for a list), or pass a value. To build a fixed number of levels by default, supply the next level's key in the builder:

# One default level of mentor, then stop
mentor: fn params -> build_user_struct(Map.put_new(params, :mentor, nil)) end

The guard applies to assocs:, FactoryMan.assoc/3,4, and FactoryMan.assoc_list/3,4. It tracks only default builds (an absent key whose default builds something), so values supplied by the caller, nested to any depth, always terminate. It cannot see why a recursion would stop, only that the same default build started again inside itself, so recursion that stops for any reason other than a supplied key is also rejected. Supply the key at each level instead.

Build or insert

assocs: builders build structs in memory unless the builder inserts. Calling insert_post/1 with a built association still persists it, because Ecto cascades the insert. The nested record is written directly by the repo, so the associated factory's before_insert and after_insert hooks do not run for it. When those hooks matter, insert the association first and pass the result, or use an insert function as the builder:

author = MyApp.Factory.Accounts.insert_user()
post = MyApp.Factory.Blog.insert_post(%{author: author})

The cascade inserts a built record once for each association that holds it. So when two belongs_to keys share one built record, for example an editor that defaults to the author:

assocs: [
  author: &Accounts.build_user_struct/1,
  editor: fn
    params, factory_params when params == %{} -> factory_params.author
    params, _factory_params -> Accounts.build_user_struct(params)
  end
]

build_post_struct/1 gives one shared %User{}, but insert_post/1 inserts two users. To share one row, insert the record first and pass it for both keys, or add a before_insert hook that inserts it once and puts the persisted record into both fields.

The same happens across a list insert: each item inserts the shared built record again.

# Each post inserts the built author again, so a unique column conflicts on the second post
MyApp.Factory.Blog.insert_post_list(2, %{author: MyApp.Factory.Accounts.build_user_struct()})

# The author is inserted once and shared by both posts
MyApp.Factory.Blog.insert_post_list(2, %{author: MyApp.Factory.Accounts.insert_user()})

Get foreign keys into params

build_*_params drops a belongs_to association and sets its foreign key only when the associated record has been inserted. With the default builder, the author is only built, so the params have no author ID:

MyApp.Factory.Blog.build_post_params()
# %{title: "post0", author_id: nil, ...}

For a changeset or controller test that needs a real foreign key, pass an inserted record:

MyApp.Factory.Blog.build_post_params(%{author: MyApp.Factory.Accounts.insert_user()})
# %{title: "post1", author_id: 42, ...}

When many tests need it, declare a variant whose builder inserts. The base factory still only builds, and the inserted author goes through the user factory's insert hooks:

defvariant persisted(params \\ %{}), for: :post, assocs: [author: &Accounts.insert_user/1] do
  params
end

MyApp.Factory.Blog.build_persisted_post_params()
# %{title: "post2", author_id: 43, ...}

Insert a dependency when the database requires it

Sometimes the schema only needs a foreign key and the related row must already exist. Make that database dependency clear in the factory:

deffactory audit_event(params \\ %{}), struct: AuditEvent do
  base_params = %{
    action: "user.created",
    user_id:
      Map.get_lazy(params, :user_id, fn ->
        MyApp.Factory.Accounts.insert_user().id
      end)
  }

  Map.merge(base_params, params)
end

Map.get_lazy/3 avoids inserting a user when the caller supplies user_id:

user = MyApp.Factory.Accounts.insert_user()
event = MyApp.Factory.insert_audit_event(%{user_id: user.id})

assert event.user_id == user.id

Use this pattern when persistence is genuinely required. For ordinary in-memory associations, build_*_struct keeps tests faster and makes the dependency smaller.

Model non-schema inputs

Factories are also useful for request payloads, job arguments, adapter options, and other values that are not structs. Omit struct: and return whatever shape the application consumes:

deffactory api_payload(params \\ %{}) do
  base_params = %{
    action: "create",
    resource: "user",
    request_id: fn -> System.unique_integer([:positive]) end
  }

  Map.merge(base_params, params)
end

deffactory request_options(overrides \\ []) do
  base_options = [
    timeout: 5_000,
    retries: 3,
    label: fn options -> "timeout-#{options[:timeout]}" end
  ]

  Keyword.merge(base_options, overrides)
end

Non-struct factories use the shorter build_* names:

payload = MyApp.Factory.build_api_payload(%{resource: "post"})
options = MyApp.Factory.build_request_options(timeout: 1_000)
payloads = MyApp.Factory.build_api_payload_list(3)

assert payload.resource == "post"
assert options[:label] == "timeout-1000"
assert length(payloads) == 3

They can return maps, keyword lists, strings, tuples, or any other value. They do not generate struct, params, or insert functions because there is no schema to provide those semantics.

Catch input mistakes with strict params

Params can be lost in two ways: a caller misspells a key, or a factory body drops a key it was given. A misspelled key in a merge-style factory normally fails later during struct construction, and a direct struct factory may ignore it entirely. A body that forgets its final merge ignores the caller's values without any error. Opt in to strict params to catch both at the factory:

deffactory user(params \\ %{}), struct: User, strict: true do
  base_params = %{
    username: FactoryMan.sequence("user"),
    email: FactoryMan.sequence(:email, fn n -> "user#{n}@example.com" end)
  }

  Map.merge(base_params, params)
end

Now a typo fails where it was introduced:

MyApp.Factory.build_user_struct(%{usernme: "alice"})
# ** (ArgumentError) unknown params [:usernme] for strict factory :user ...

A body that ignores or changes a param it was given fails too:

deffactory user(params \\ %{}), struct: User, strict: true do
  %{username: FactoryMan.sequence("user")}
end

MyApp.Factory.build_user_struct(%{username: "alice"})
# ** (ArgumentError) strict factory :user in MyApp.Factory ignored or changed params it was given:
#
#      :username - given "alice", built "user0"
#    ...

Strict validation also applies through params builders, inserts, list builders, and variants. Turn it on in the base factory, so every child factory is strict unless one overrides the option:

defmodule MyApp.Factory do
  use FactoryMan, repo: MyApp.Repo, strict: true
end

A factory may intentionally accept an input that is not a struct field. Allow that input explicitly:

deffactory user_from_domain(params \\ %{}),
  struct: User,
  body: :struct,
  strict: [allow: [:domain]] do
  domain = Map.get(params, :domain, "example.com")

  %User{
    username: FactoryMan.sequence("derived-user"),
    email: "derived@#{domain}"
  }
end

allow: also covers a field that the body changes on purpose, such as a value it normalizes:

deffactory account(params \\ %{}), struct: Account, strict: [allow: [:email]] do
  %{email: "user@example.com"}
  |> Map.merge(params)
  |> Map.update!(:email, &String.downcase/1)
end

Keys in allow: are not checked at all. Other keys outside the struct fields still raise, and other fields must still come through the body unchanged.

Organize a growing factory suite

A single factory module is convenient at first. As the application grows, keep shared configuration in a small parent module and organize child factories around application contexts.

test/support/
  factory.ex                    # repo, shared hooks, shared helpers
  factory/
    accounts.ex                 # user and account factories
    blog.ex                     # post and tag factories
    blog/comments.ex            # comment factories

Share configuration with extends:

The parent owns configuration that should be consistent across the suite:

defmodule MyApp.Factory do
  use FactoryMan,
    repo: MyApp.Repo,
    hooks: [after_insert: &__MODULE__.reset_associations/1]

  def reset_associations(%_{} = struct) do
    Ecto.reset_fields(struct, struct.__struct__.__schema__(:associations))
  end
end

Child modules inherit the repo, hooks, and public helper functions:

defmodule MyApp.Factory.Accounts do
  use FactoryMan, extends: MyApp.Factory

  alias MyApp.Accounts.User

  deffactory user(params \\ %{}), struct: User do
    base_params = %{username: FactoryMan.sequence("user")}

    Map.merge(base_params, params)
  end
end

The after_insert hook resets loaded associations so an inserted result resembles a record returned by a fresh query. This prevents tests from accidentally depending on associations that happened to be present during construction.

Inheritance chains may have more than one level, and a child may override inherited options:

defmodule MyApp.Factory.Reporting do
  # Inherits the hooks, and uses another repo
  use FactoryMan, extends: MyApp.Factory, repo: MyApp.ReportingRepo
end

defmodule MyApp.Factory.Reporting.Exports do
  # Inherits MyApp.Factory.Reporting's resolved options, including its repo
  use FactoryMan, extends: MyApp.Factory.Reporting
end

Keep the parent focused on shared behavior; domain-specific factory definitions belong in the child modules that use them.

Switch off functions the suite does not use

A suite that never calls some generated functions can switch them off in the base factory. Fewer functions keep autocompletion and the generated docs focused:

defmodule MyApp.Factory do
  use FactoryMan,
    repo: MyApp.Repo,
    disable: [string_params: true, params_list: true]
end

A factory that needs one of them back enables it again with false:

deffactory registration(params \\ %{}), struct: Registration, disable: [string_params: false] do
  Map.merge(%{email: FactoryMan.sequence(:email, &"user#{&1}@example.com")}, params)
end

build_*_struct is always generated, and inserts are switched off with insert: false. Check what a factory has disabled with __factory_man__(:opts, :registration)[:disable].

Use hooks for cross-cutting behavior

Hooks transform values at defined points in the build and insert pipeline. A factory-local hook is useful when a rule belongs to one kind of data:

defmodule MyApp.Factory.Events do
  use FactoryMan

  def add_test_source(params), do: Map.put_new(params, :source, "test")

  deffactory event(params \\ %{}),
    hooks: [before_build_params: &__MODULE__.add_test_source/1] do
    base_params = %{name: "user.created"}

    Map.merge(base_params, params)
  end
end

A module-level hook is better for behavior shared by every factory in that module. Hooks must be remote captures (&__MODULE__.my_hook/1); anything else, such as an anonymous function, raises at compile time. To run several functions at one hook, give a list; they run in order:

hooks: [after_insert: [&__MODULE__.reset_associations/1, &__MODULE__.log_insert/1]]

For a normal struct factory, the build path is:

params validation
→ before_build_params
→ assocs: resolution
→ factory body and lazy evaluation
→ strict params check (with strict: true)
→ after_build_params
→ before_build_struct
→ struct!/2
→ after_build_struct

An insert continues with before_insert, the repo insert, and after_insert. This is also why insert_*_struct is preferable to a direct repo call after editing a built struct: it keeps the insert hooks in the path.

Chain a factory hook with an inherited one

A hook set on a factory is chained with the hooks it inherits from its modules. The order is onion-style, like middleware: the parent wraps the child. A parent's before_* hook runs first, and a parent's after_* hook runs last.

With the reset_associations/1 parent from "Share configuration with extends:", a factory-level after_insert runs before the reset. In the MyApp.Factory.Blog module from above:

deffactory post(params \\ %{}),
  struct: Post,
  hooks: [after_insert: &__MODULE__.add_comment/1] do
  base_params = %{title: FactoryMan.sequence("post")}

  Map.merge(base_params, params)
end

def add_comment(post) do
  insert_comment(%{post: post})
  post
end

insert_post/0 runs add_comment/1, then reset_associations/1, so the result still resembles a freshly queried record. To run a hook after the inherited ones instead (for example, to preload the comments onto the result), place it explicitly:

hooks: [after_insert: {&__MODULE__.preload_comments/1, :after_parent}]

The placements are :before_parent, :after_parent, and :replace_parent. A placement applies to a whole list of hooks. To switch off the inherited hooks for one factory, replace them with an empty list:

hooks: [after_insert: {[], :replace_parent}]

__factory_man__(:opts, :post) shows each hook name's resolved list in run order.

Cast values before insert

FactoryMan inserts values exactly as the factory builds them, so a value of the wrong type (e.g. "42" for an integer field) fails in the repo. To convert values to their field types first, as a changeset would, cast them in a before_insert hook:

defmodule MyApp.Factory do
  use FactoryMan, repo: MyApp.Repo, hooks: [before_insert: &__MODULE__.cast_fields/1]

  def cast_fields(%schema{} = struct) do
    fields = schema.__schema__(:fields) -- schema.__schema__(:embeds)

    schema
    |> struct()
    |> Ecto.Changeset.cast(Map.take(struct, fields), fields, empty_values: [])
    |> Ecto.Changeset.apply_action!(:insert)
    |> then(&Map.merge(struct, Map.take(&1, fields)))
  end
end

A value that cannot be cast raises an Ecto.InvalidChangesetError that names the field. The hook casts the struct's own fields only, not its associations or embeds. It does not run your schema's changeset/2 (which would also validate): a required foreign key that an association fills in on insert would fail validation before the insert.

Handle specialized construction

Most factories should return params maps. The following tools are useful when the value does not follow the ordinary table-backed Ecto path.

Build embedded schemas

Embedded schemas use the normal struct and params builders. They cannot be inserted with Ecto, so FactoryMan generates no Ecto insert functions for them:

defmodule MyApp.Factory.Settings do
  use FactoryMan, extends: MyApp.Factory

  alias MyApp.Accounts.Settings

  deffactory settings(params \\ %{}), struct: Settings do
    base_params = %{
      theme: "dark",
      notifications: true
    }

    Map.merge(base_params, params)
  end
end

Use the result in tests just like another in-memory struct:

settings = MyApp.Factory.Settings.build_settings_struct(%{theme: "light"})
attrs = MyApp.Factory.Settings.build_settings_params()

assert settings.theme == "light"
assert attrs.notifications

There is no insert_settings because an embedded schema has no table of its own. To store one somewhere else, give it an insert function (see Insert into other stores).

Return a struct directly with body: :struct

Use body: :struct when construction is genuinely easier after another struct has been built, or when the body needs control that a params map cannot express:

deffactory anonymized_user(params \\ %{}), struct: User, body: :struct do
  user = build_user_struct(params)

  %{user |
    username: "anonymous",
    email: "redacted@example.com",
    display_name: "Anonymous user"
  }
end

The full function family is still generated:

MyApp.Factory.build_anonymized_user_struct()
MyApp.Factory.build_anonymized_user_params()
MyApp.Factory.build_anonymized_user_struct_list(3)
MyApp.Factory.insert_anonymized_user()

Params-stage hooks are skipped because the body does not perform params-to-struct conversion. Lazy values in the returned struct are still resolved, and after_build_struct and insert hooks still run. In the wrapper above, an after_build_struct hook runs once inside build_user_struct/1 and again for the wrapping factory.

Use direct struct bodies sparingly. A normal params body is easier to extend, compose, and inspect.

Insert into other stores

By default, insert_* inserts with the repo. An insert target sends a built struct somewhere else: a search index, a cache, an in-memory store, or an external API stub.

Insert a plain struct

A plain struct has no Ecto insert. Give it one with insert:, a remote capture of arity 2:

defmodule MyApp.Factory.Events do
  use FactoryMan, extends: MyApp.Factory

  alias MyApp.Events.Event

  deffactory event(params \\ %{}), struct: Event, insert: &__MODULE__.publish!/2 do
    base_params = %{name: FactoryMan.sequence("event"), payload: %{}}

    Map.merge(base_params, params)
  end

  def publish!(event, _opts) do
    :ok = MyApp.EventBus.publish(event)

    event
  end
end

The whole insert family is generated, and the insert hooks run around publish!/2:

MyApp.Factory.Events.insert_event(%{name: "signed_up"})
MyApp.Factory.Events.insert_event_list(3)
MyApp.Factory.Events.insert_event_struct(event)

An insert function receives the built struct and the caller's options (without :variants), and returns the inserted struct. A store that takes no options ignores them, as above.

Add a named target

insert_via: keeps the Ecto insert and adds a second way to insert. Set it in the base factory to give every factory the target:

defmodule MyApp.Factory do
  use FactoryMan,
    repo: MyApp.Repo,
    insert_via: [search: &__MODULE__.index!/2]

  def index!(struct, opts) do
    MyApp.Search.index!(struct, refresh: Keyword.get(opts, :refresh, true))

    struct
  end
end

Each struct factory now has an insert_*_via_search family:

user = insert_user_via_search(%{username: "alice"})
users = insert_user_via_search_list(3, %{}, refresh: false)
post = insert_post_via_search(%{}, variants: [:published])

insert_via: merges by name: a factory adds its own targets, replaces an inherited one with the same name, or removes it with false:

deffactory audit_log(params \\ %{}), struct: AuditLog, insert_via: [search: false] do
  Map.merge(%{action: "login"}, params)
end

Targets run no insert hooks, and they do not check whether the struct has already been inserted. Anything a target needs goes inside its function.

Insert a row, then index it

A row that a query needs and a search document that a search needs are two steps. Pipe the inserted row into the target's struct form:

user =
  %{username: "alice"}
  |> insert_user()
  |> insert_user_struct_via_search()

insert_user/1 runs the insert hooks and the Ecto insert. insert_user_struct_via_search/1 indexes the record it is given, with its database id.

Do both steps in one target

When most tests need both, make one target do both:

use FactoryMan,
  repo: MyApp.Repo,
  insert_via: [indexed: &__MODULE__.insert_and_index!/2]

def insert_and_index!(struct, opts) do
  struct
  |> MyApp.Repo.insert!(opts)
  |> tap(&MyApp.Search.index!(&1, refresh: true))
end
user = insert_user_via_indexed(%{username: "alice"})

The target calls MyApp.Repo.insert!/2 directly, so the factory's insert hooks do not run. To get the same result shape as insert_user/1, call your hook functions from the target.

Check where a factory inserts

The resolved targets appear in the factory's options:

iex> MyApp.Factory.Accounts.__factory_man__(:opts, :user)[:insert]
:ecto

iex> MyApp.Factory.Accounts.__factory_man__(:opts, :user)[:insert_via]
[search: &MyApp.Factory.index!/2]

Build presets that keep their promises

Variants are ideal for caller-overridable defaults. Three more specialized patterns help when a preset must force a value, transform a finished value, or enforce an invariant.

Forced values

When a preset's name is a promise that a caller should not be able to break by accident, merge the preset's values over the params instead of under them:

defvariant banned(params \\ %{}), for: :user do
  Map.merge(params, %{banned: true})
end

MyApp.Factory.build_banned_user_struct(%{banned: false}).banned
# true

The forced value also wins over every variant after it in a variants: list. Use this form sparingly: a caller cannot override the value, even on purpose.

Post-build presets

A variant cannot transform the finished struct because it runs before the base factory. Wrap the base builder in a direct struct factory when the transformation belongs after construction:

deffactory verified_user(params \\ %{}), struct: User, body: :struct do
  user = build_user_struct(params)

  %{user | verified_at: DateTime.utc_now()}
end

This retains params, list, and insert functions under the new factory name. Remember the after_build_struct double-run caveat when the wrapper calls another factory that has the same hook.

Validated presets

A variant's defaults may be overridden. If a name promises a property such as “published,” validate the merged params before delegating to the base factory:

defvariant published(params \\ %{}), for: :post do
  base_params = %{
    published_at: DateTime.utc_now(),
    draft: false
  }

  result_params = Map.merge(base_params, params)

  if result_params[:draft] or is_nil(result_params[:published_at]) do
    raise ArgumentError, "published posts require published_at and draft: false"
  end

  result_params
end

A contradictory call now fails at the factory boundary:

MyApp.Factory.Blog.build_published_post_struct(%{draft: true})
# ** (ArgumentError) published posts require published_at and draft: false

The validation sees raw params before lazy evaluation. Do not write a predicate that expects a function-valued field to have been resolved already.

Inspect factories when names are dynamic

Most tests should call generated functions directly. Reflection is useful for test helpers or tools that receive a factory name at runtime:

def build_named(factory_module, factory_name, params \\ %{}) do
  unless factory_name in factory_module.__factory_man__(:factories) do
    raise ArgumentError, "unknown factory #{inspect(factory_name)}"
  end

  if is_nil(factory_module.__factory_man__(:opts, factory_name)[:struct]) do
    raise ArgumentError, "factory #{inspect(factory_name)} does not build a struct"
  end

  apply(factory_module, :"build_#{factory_name}_struct", [params])
end

Checking __factory_man__(:factories) before constructing the function name limits dispatch to registered factories. Variants appear under their full registered names. __factory_man__(:variants, :user) lists a factory's variants by the names that variants: accepts, e.g. to build every variant of a factory in a test.

Habits that keep factories easy to use

The usage rules list the rules and anti-patterns for writing factories. The FactoryMan module documentation is the full reference for edge cases.