Sample

Navigate default foreign key types in Ecto associations and handle :id type defaults when schemas use binary_id primary keys

sha256:e447eb9ad840ecdae7ce357052059a87dbbe2a677cede495fd997434da32bf90

Case

Goal

Navigate default foreign key types in Ecto associations and handle :id type defaults when schemas use binary_id primary keys HOW

Packages

ecto 3.14.1

Environment

elixir

Created

2026-08-17T04:43:50Z

Commonly assumed

Declaring @primary_key {:id, :binary_id, autogenerate: true} on an Ecto schema sets the default foreign key type for belongs_to associations to :binary_id, allowing UUID string foreign key parameters to be cast without explicit type configuration.

The sample's author recorded this as what a developer or model would expect here. The contract below is what actually ran.

Contract

• assert belongs_to defaults the foreign key field type to :id integer even when @primary_key is configured as :binary_id, causing binary UUID string parameters to fail with a cast validation error
• assert casting integer values into default belongs_to foreign key fields succeeds because the unconfigured foreign key type defaults to :id
• assert configuring @foreign_key_type :binary_id sets the default association foreign key field type to :binary_id and allows valid UUID strings to cast successfully
• assert belongs_to defaults :foreign_key to the association name with an _id suffix and :references to :id
• assert belongs_to with define_field: false constructs the association reflection but omits the foreign key from schema fields
• assert has_many and has_one default :foreign_key to the snake_cased source module name with an _id suffix and :references to :id

Files

• NOTES.md
• csx.json
• mix.exs
• mix.lock
• test/contract.exs
• test/test_helper.exs

Download the source artifact (tar.gz)

Origin Seeder

csx-seed

Verification receipts

• elixir 1 · linux alpine/x64 · docker · CONTAINER_RUN · compile:PASS · contract:PASS · load:PASS · resolve:PASS · hex@1 · 2026-08-17 · ed25519:d91480838ac982c9