Stateful in-memory Repo fake (closed-world). Recommended default.

The state is the complete truth — if a record isn't in the store, it doesn't exist. This makes the adapter authoritative for all bare schema operations without needing a fallback function.

Implements DoubleDown.Contract.Dispatch.FakeHandler, so it can be used by module name with Double.fake:

Usage with Double.fake

# Basic — all bare-schema reads work without fallback:
DoubleDown.Double.fake(DoubleDown.Repo, DoubleDown.Repo.InMemory)

# With seed data:
DoubleDown.Double.fake(DoubleDown.Repo, DoubleDown.Repo.InMemory,
  [%User{id: 1, name: "Alice"}, %Post{id: 1, title: "Hello"}])

# Layer expects for failure simulation:
DoubleDown.Repo
|> DoubleDown.Double.fake(DoubleDown.Repo.InMemory)
|> DoubleDown.Double.expect(:insert, fn [changeset] ->
  {:error, Ecto.Changeset.add_error(changeset, :email, "taken")}
end)

ExMachina integration

setup do
  DoubleDown.Double.fake(DoubleDown.Repo, DoubleDown.Repo.InMemory)
  insert(:user, name: "Alice", email: "alice@example.com")
  insert(:user, name: "Bob", email: "bob@example.com")
  :ok
end

test "lists all users" do
  assert [_, _] = MyApp.Repo.all(User)
end

test "finds user by email" do
  assert %User{name: "Alice"} =
    MyApp.Repo.get_by(User, email: "alice@example.com")
end

test "count users" do
  assert 2 = MyApp.Repo.aggregate(User, :count, :id)
end

Authoritative operations (bare schema queryables)

Ecto.Query fallback

Operations with Ecto.Query queryables (containing where, join, select etc.) cannot be evaluated in-memory. These fall through to the fallback function, or raise with a clear error:

DoubleDown.Double.fake(DoubleDown.Repo, DoubleDown.Repo.InMemory, [],
  fallback_fn: fn
    _contract, :all, [%Ecto.Query{}], _state -> []
  end
)

When to use which Repo fake

Limitations

The following insert_all options are silently ignored because they depend on database constraints that don't exist in memory:

• on_conflict — upsert behaviour (:nothing, {:replace, fields}, etc.)
• conflict_target — which fields/index define the conflict

This is intentional: test code should match production code without modification. Testing constraint behaviour requires a real database via DataCase.

When returning: is a list of fields, insert_all returns maps containing only those fields (matching Ecto adapter behaviour). returning: true returns full structs.

Binary table name sources (e.g. "users") are not supported by insert_all — use a fallback function or Double.expect for these.

See also

• DoubleDown.Repo.OpenInMemory — open-world variant where absence is inconclusive, requiring a fallback for most reads.
• DoubleDown.Repo.Stub — stateless stub for fire-and-forget writes.