I’ve been trying to get foreign_keys to work with string id’s for days and still have not been able to figure it out.

I am stuck with this error:

iex(3)> project_manager = Repo.all(from p in ProjectManager, preload: [:user])
[debug] QUERY OK source="project_managers" db=0.3ms
SELECT p0."id", p0."wf_user_id", p0."inserted_at", p0."updated_at" FROM "project_managers" AS p0 []
** (Ecto.Query.CastError) deps/ecto/lib/ecto/association.ex:901: value `["5b89e3d300abc77aa4885e819fe3713d", "5b93028601bd041b925dc2067422be82", "5ba94a2a00854529705f809ebec755b9"]` in `where` cannot be cast to type {:in, :id} in query:

from u0 in ExampleApp.Users.User,
  where: u0.id in ^["5b89e3d300abc77aa4885e819fe3713d", "5b93028601bd041b925dc2067422be82", "5ba94a2a00854529705f809ebec755b9"],
  select: {u0.id, u0}

    (elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
    (elixir) lib/enum.ex:1440: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
    (elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
    (ecto) lib/ecto/repo/queryable.ex:161: Ecto.Repo.Queryable.execute/4
    (ecto) lib/ecto/repo/queryable.ex:17: Ecto.Repo.Queryable.all/3
    (elixir) lib/enum.ex:1336: Enum."-map/2-lists^map/1-0-"/2

I distilled to minimum reproduction of the issue, documented the steps and put up on a repo.

Repo:
https://github.com/bigbassroller/Elixir-Phoenix-Foreign-Key-Example

Here are the steps to reproduce (taken from the README of the repo).

ExampleApp

First, make sure you have Elixir and Phoenix installed:

• https://hexdocs.pm/phoenix/installation.html

Also make sure you have PostgreSQL installed:

• https://wiki.postgresql.org/wiki/Detailed_installation_guides

mix phx.new example_app

cd example_app

mix ecto.create

mix phx.server

Add User Schema

mix phx.gen.schema Users.User users wf_user_id:string:unique email:string:unique 

Add Project Manager Schema

mix phx.gen.schema ObjectTypes.ProjectManager project_managers wf_user_id:references:users:unique

Add Belongs to User to Project Manager

lib/example_app/object_types/project_manager.ex

defmodule ExampleApp.ObjectTypes.ProjectManager do
  use Ecto.Schema
  import Ecto.Changeset
  alias ExampleApp.Users.User
  # use ExampleApp.Schema


  schema "project_managers" do
    belongs_to :user, User, foreign_key: :wf_user_id, type: :string, primary_key: true

    timestamps()
  end

  @doc false
  def changeset(project_manager, attrs) do
    project_manager
    |> cast(attrs, [])
    |> validate_required([])
    |> unique_constraint(:wf_user_id)
  end
end

Set schema primary keys and foreign key type

Also tried with this, but still no dice:(

lib/example_app/schema.ex

defmodule ExampleApp.Schema do
  defmacro __using__(_) do
    quote do
      use Ecto.Schema
      @primary_key {:wf_user_id, :string, autogenerate: false}
      @foreign_key_type :string
    end
  end
end

priv/repo/migrations/20191208072158_create_users.exs

defmodule ExampleApp.Repo.Migrations.CreateUsers do
  use Ecto.Migration

  def change do
    create table(:users) do
      add :wf_user_id, :string, primary_key: true
      add :email, :string

      timestamps()
    end

    create unique_index(:users, [:wf_user_id])
    create unique_index(:users, [:email])
  end
end

priv/repo/migrations/20191208072217_create_project_managers.exs

defmodule ExampleApp.Repo.Migrations.CreateProjectManagers do
  use Ecto.Migration

  def change do
    create table(:project_managers) do
      add :wf_user_id, references(:users, column: :wf_user_id, type: :string, on_delete: :delete_all), null: false, primary_key: true

      timestamps()
    end

    create unique_index(:project_managers, [:wf_user_id])
  end
end

Customize iex

.iex.exs

import_if_available Ecto.Query

alias ExampleApp.{
    Repo,
    Users.User,
    ObjectTypes,
    ObjectTypes.Project,
    ObjectTypes.ProjectManager
}

Seed User table with Mock Data from JSON response

priv/repo/user-seeds.exs

# Script for populating the database. You can run it as:
#
#     mix run priv/repo/user-seeds.exs

alias ExampleApp.Repo
alias ExampleApp.Users.User

# ID is from legacy system
data = [
  %{
    "ID" => "5ba94a2a00854529705f809ebec755b9",
    "emailAddr" => "user1@test.com",
  },
  %{
    "ID" => "5b93028601bd041b925dc2067422be82",
    "emailAddr" => "user2@test.com",
  },
  %{
    "ID" => "5b89e3d300abc77aa4885e819fe3713d",
    "emailAddr" => "user3@test.com",
  }
]

Enum.each data, fn(user) ->

  # Map Data here
  wf_user_id = Map.get(user, "ID")
  email_addr = Map.get(user, "emailAddr")

  # Insert data into database
  Repo.insert! %User{
    wf_user_id: wf_user_id,
    email: email_addr
  }
end

mix run priv/repo/user-seeds.exs

Seed Project Manager table with Mock Data from JSON response

priv/repo/project-manager-seeds.exs

# Script for populating the database. You can run it as:
#
#     mix run priv/repo/project-manager-seeds.exs

alias ExampleApp.Repo
alias ExampleApp.ObjectTypes.{ProjectManager}

data = [
  %{
    "ID" => "5ba94b28008708718d6e1b4d36a79770",
    "name" => "Project 1",
    "ownerID" => "5ba94a2a00854529705f809ebec755b9"
  },
  %{
    "ID" => "5bae225c02b878793ca699dc8a4b9b9a",
    "name" => "Project 2",
    "ownerID" => "5b93028601bd041b925dc2067422be82"
  },
  %{
    "ID" => "5bc8ae5a00251a1cb4ee27c4860e29b1",
    "name" => "Project 3",
    "ownerID" => "5b89e3d300abc77aa4885e819fe3713d"
  }
]

Enum.each data, fn(project_manager) ->

  wf_user_id = Map.get(project_manager, "ownerID")

  # Insert data into database
  Repo.insert! %ProjectManager{
    wf_user_id: wf_user_id
  }
end

mix run priv/repo/project-manager-seeds.exs

Do a reset and reseed:

mix ecto.reset && mix run priv/repo/user-seeds.exs \
&& mix run priv/repo/project-manager-seeds.exs

Test it out:

iex -S mix

users = Repo.all(from u in User)

Outputs:

[debug] QUERY OK source="users" db=5.9ms decode=1.0ms queue=1.3ms
SELECT u0."id", u0."email", u0."wf_user_id", u0."inserted_at", u0."updated_at" FROM "users" AS u0 []
[
  %ExampleApp.Users.User{
    __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
    email: "user1@test.com",
    id: 1,
    inserted_at: ~N[2019-12-08 08:00:10],
    updated_at: ~N[2019-12-08 08:00:10],
    wf_user_id: "5ba94a2a00854529705f809ebec755b9"
  },
  %ExampleApp.Users.User{
    __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
    email: "user2@test.com",
    id: 2,
    inserted_at: ~N[2019-12-08 08:00:10],
    updated_at: ~N[2019-12-08 08:00:10],
    wf_user_id: "5b93028601bd041b925dc2067422be82"
  },
  %ExampleApp.Users.User{
    __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
    email: "user3@test.com",
    id: 3,
    inserted_at: ~N[2019-12-08 08:00:10],
    updated_at: ~N[2019-12-08 08:00:10],
    wf_user_id: "5b89e3d300abc77aa4885e819fe3713d"
  }
]

project_manager = Repo.all(from p in ProjectManager)

Outputs:

[debug] QUERY OK source="project_managers" db=1.4ms queue=1.4ms
SELECT p0."id", p0."wf_user_id", p0."inserted_at", p0."updated_at" FROM "project_managers" AS p0 []
[
  %ExampleApp.ObjectTypes.ProjectManager{
    __meta__: #Ecto.Schema.Metadata<:loaded, "project_managers">,
    id: 1,
    inserted_at: ~N[2019-12-08 08:00:11],
    updated_at: ~N[2019-12-08 08:00:11],
    user: #Ecto.Association.NotLoaded<association :user is not loaded>,
    wf_user_id: "5ba94a2a00854529705f809ebec755b9"
  },
  %ExampleApp.ObjectTypes.ProjectManager{
    __meta__: #Ecto.Schema.Metadata<:loaded, "project_managers">,
    id: 2,
    inserted_at: ~N[2019-12-08 08:00:11],
    updated_at: ~N[2019-12-08 08:00:11],
    user: #Ecto.Association.NotLoaded<association :user is not loaded>,
    wf_user_id: "5b93028601bd041b925dc2067422be82"
  },
  %ExampleApp.ObjectTypes.ProjectManager{
    __meta__: #Ecto.Schema.Metadata<:loaded, "project_managers">,
    id: 3,
    inserted_at: ~N[2019-12-08 08:00:11],
    updated_at: ~N[2019-12-08 08:00:11],
    user: #Ecto.Association.NotLoaded<association :user is not loaded>,
    wf_user_id: "5b89e3d300abc77aa4885e819fe3713d"
  }
]

project_manager = Repo.all(from p in ProjectManager, preload: [:user])

Roh oh! What does {:in, :id} mean?

iex(3)> project_manager = Repo.all(from p in ProjectManager, preload: [:user])
[debug] QUERY OK source="project_managers" db=0.3ms
SELECT p0."id", p0."wf_user_id", p0."inserted_at", p0."updated_at" FROM "project_managers" AS p0 []
** (Ecto.Query.CastError) deps/ecto/lib/ecto/association.ex:901: value `["5b89e3d300abc77aa4885e819fe3713d", "5b93028601bd041b925dc2067422be82", "5ba94a2a00854529705f809ebec755b9"]` in `where` cannot be cast to type {:in, :id} in query:

from u0 in ExampleApp.Users.User,
  where: u0.id in ^["5b89e3d300abc77aa4885e819fe3713d", "5b93028601bd041b925dc2067422be82", "5ba94a2a00854529705f809ebec755b9"],
  select: {u0.id, u0}

    (elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
    (elixir) lib/enum.ex:1440: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
    (elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
    (ecto) lib/ecto/repo/queryable.ex:161: Ecto.Repo.Queryable.execute/4
    (ecto) lib/ecto/repo/queryable.ex:17: Ecto.Repo.Queryable.all/3
    (elixir) lib/enum.ex:1336: Enum."-map/2-lists^map/1-0-"/2

Expected output is something like this:

[
  %ExampleApp.ObjectTypes.ProjectManager{
    __meta__: #Ecto.Schema.Metadata<:loaded, "project_managers">,
    id: 1,
    inserted_at: ~N[2019-12-05 17:20:11],
    projects: #Ecto.Association.NotLoaded<association :projects is not loaded>,
    updated_at: ~N[2019-12-05 17:20:11],
    user: %ExampleApp.Users.User{
      __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
      email: "user1@test.com",
      id: 1,
      inserted_at: ~N[2019-12-05 17:20:10],
      updated_at: ~N[2019-12-05 17:20:10],
      wf_user_id: "5ba94a2a00854529705f809ebec755b9"
    },
    wf_user_id: "5ba94a2a00854529705f809ebec755b9"
  },
  %ExampleApp.ObjectTypes.ProjectManager{
    __meta__: #Ecto.Schema.Metadata<:loaded, "project_managers">,
    id: 2,
    inserted_at: ~N[2019-12-05 17:20:11],
    projects: #Ecto.Association.NotLoaded<association :projects is not loaded>,
    updated_at: ~N[2019-12-05 17:20:11],
    user: %ExampleApp.Users.User{
      __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
      email: "user2@test.com",
      id: 2,
      inserted_at: ~N[2019-12-05 17:20:10],
      updated_at: ~N[2019-12-05 17:20:10],
      wf_user_id: "5b93028601bd041b925dc2067422be82"
    },
    wf_user_id: "5ba94a2a00854529705f809ebec755b9"
  },
  %ExampleApp.ObjectTypes.ProjectManager{
    __meta__: #Ecto.Schema.Metadata<:loaded, "project_managers">,
    id: 3,
    inserted_at: ~N[2019-12-05 17:20:11],
    projects: #Ecto.Association.NotLoaded<association :projects is not loaded>,
    updated_at: ~N[2019-12-05 17:20:11],
    user: %ExampleApp.Users.User{
      __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
      email: "user3@test.com",
      id: 3,
      inserted_at: ~N[2019-12-05 17:20:10],
      updated_at: ~N[2019-12-05 17:20:10],
      wf_user_id: "5b89e3d300abc77aa4885e819fe3713d"
    },
    wf_user_id: "5b89e3d300abc77aa4885e819fe3713d"
  }
]

This works though

q = from ProjectManager, where: [wf_user_id: "5b93028601bd041b925dc2067422be82"]
Repo.all(q)

iex(1)> q = from ProjectManager, where: [wf_user_id: "5b93028601bd041b925dc2067422be82"]
#Ecto.Query<from p0 in ExampleApp.ObjectTypes.ProjectManager,
 where: p0.wf_user_id == "5b93028601bd041b925dc2067422be82">
iex(2)> Repo.all(q)
[debug] QUERY OK source="project_managers" db=6.1ms decode=1.0ms queue=2.9ms
SELECT p0."id", p0."wf_user_id", p0."inserted_at", p0."updated_at" FROM "project_managers" AS p0 WHERE (p0."wf_user_id" = '5b93028601bd041b925dc2067422be82') []
[
  %ExampleApp.ObjectTypes.ProjectManager{
    __meta__: #Ecto.Schema.Metadata<:loaded, "project_managers">,
    id: 2,
    inserted_at: ~N[2019-12-08 08:00:11],
    updated_at: ~N[2019-12-08 08:00:11],
    user: #Ecto.Association.NotLoaded<association :user is not loaded>,
    wf_user_id: "5b93028601bd041b925dc2067422be82"
  }
]

Users Table Output

                                         Table "public.users"
   Column    |              Type              | Collation | Nullable |              Default              
-------------+--------------------------------+-----------+----------+-----------------------------------
 id          | bigint                         |           | not null | nextval('users_id_seq'::regclass)
 wf_user_id  | character varying(255)         |           | not null | 
 email       | character varying(255)         |           |          | 
 inserted_at | timestamp(0) without time zone |           | not null | 
 updated_at  | timestamp(0) without time zone |           | not null | 
Indexes:
    "users_pkey" PRIMARY KEY, btree (id, wf_user_id)
    "users_email_index" UNIQUE, btree (email)
    "users_wf_user_id_index" UNIQUE, btree (wf_user_id)
Referenced by:
    TABLE "project_managers" CONSTRAINT "project_managers_wf_user_id_fkey" FOREIGN KEY (wf_user_id) REFERENCES users(wf_user_id) ON DELETE CASCADE

 id |            wf_user_id            |     email      |     inserted_at     |     updated_at      
----+----------------------------------+----------------+---------------------+---------------------
  1 | 5ba94a2a00854529705f809ebec755b9 | user1@test.com | 2019-12-08 08:00:10 | 2019-12-08 08:00:10
  2 | 5b93028601bd041b925dc2067422be82 | user2@test.com | 2019-12-08 08:00:10 | 2019-12-08 08:00:10
  3 | 5b89e3d300abc77aa4885e819fe3713d | user3@test.com | 2019-12-08 08:00:10 | 2019-12-08 08:00:10
(3 rows)

Project Managers Table Output

                                          Table "public.project_managers"
   Column    |              Type              | Collation | Nullable |                   Default                    
-------------+--------------------------------+-----------+----------+----------------------------------------------
 id          | bigint                         |           | not null | nextval('project_managers_id_seq'::regclass)
 wf_user_id  | character varying(255)         |           | not null | 
 inserted_at | timestamp(0) without time zone |           | not null | 
 updated_at  | timestamp(0) without time zone |           | not null | 
Indexes:
    "project_managers_pkey" PRIMARY KEY, btree (id, wf_user_id)
    "project_managers_wf_user_id_index" UNIQUE, btree (wf_user_id)
Foreign-key constraints:
    "project_managers_wf_user_id_fkey" FOREIGN KEY (wf_user_id) REFERENCES users(wf_user_id) ON DELETE CASCADE

 id |            wf_user_id            |     inserted_at     |     updated_at      
----+----------------------------------+---------------------+---------------------
  1 | 5ba94a2a00854529705f809ebec755b9 | 2019-12-08 08:00:11 | 2019-12-08 08:00:11
  2 | 5b93028601bd041b925dc2067422be82 | 2019-12-08 08:00:11 | 2019-12-08 08:00:11
  3 | 5b89e3d300abc77aa4885e819fe3713d | 2019-12-08 08:00:11 | 2019-12-08 08:00:11
(3 rows)

Any idea what I am missing?

Thanks in advance for any help!