Getting Ecto to pass foreign keys - sql

Question

I'm working with Ecto (with Postgres) for the first time and I have the following two schemas (both somewhat simplified):
defmodule RailroadServer.Database.RailroadSystem do
#moduledoc """
Schema for an entire railroad system.
"""
use Ecto.Schema
import Ecto.Changeset
alias RailroadServer.Database
schema "railroad_systems" do
field :railroad_system_name, :string
has_many :depos, Database.Depo
end
#fields ~w(railroad_system_name)a
def changeset(data, params \\ %{}) do
data
|> cast(params, #fields)
|> validate_required([:railroad_system_name])
|> validate_length(:railroad_system_name, max: 50)
end
end
defmodule RailroadServer.Database.Depo do
#moduledoc """
Schema for a node that stores trains.
"""
use Ecto.Schema
import Ecto.Changeset
alias RailroadServer.Database
schema "depos" do
field :capacity, :integer
field :depo_uuid, :string
field :depo_name, :string
belongs_to :railroad_system, Database.RailroadSystem
end
#fields ~w(capacity depo_uuid depo_name)a
def changeset(data, params \\ %{}) do
data
|> cast(params, #fields)
|> validate_required([:capacity, :depo_uuid, :depo_name])
|> validate_number(:capacity, greater_than: 0)
|> validate_length(:depo_name, max: 50)
|> validate_length(:depo_uuid, max: 50)
|> foreign_key_constraint(:railroad_system_id)
end
end
Based on these migrations:
defmodule RailroadServer.Database.Repo.Migrations.CreateRailroadSystems do
use Ecto.Migration
def change do
create table(:railroad_systems) do
add :railroad_system_name, :varchar, null: false, size: 50
end
create unique_index("railroad_systems", [:railroad_system_name])
end
end
defmodule RailroadServer.Database.Repo.Migrations.CreateDepos do
use Ecto.Migration
def change do
create table(:depos) do
add :railroad_system_id, references("railroad_systems"), null: false
add :depo_uuid, :varchar, size: 50, null: false
add :depo_name, :varchar, size: 50, null: false
add :capacity, :integer, null: false
end
create index("depos", [:railroad_system_id])
create index("depos", [:depo_uuid], unique: true)
create index("depos", [:depo_name], unique: true)
end
end
Which I'm constructing with the following code:
def insert_railway_system(system_name, depos) do
cs = %RailroadSystem{}
|> RailroadSystem.changeset(%{railroad_system_name: system_name})
|> put_assoc(:depos, create_depos(depos))
if cs.valid? do
Repo.insert(cs)
else
{:error, cs}
end
end
_ = """
Uses a list of depo nodes to construct a list of depo changeset.
"""
defp create_depos(depos) do
Enum.map(depos, fn(depo) -> Depo.changeset(%Depo{}, depo) end)
end
However, when I run this function (with data that produces a valid changeset), I get a NULL column error because the foreign key for the railway system in the depo struct doesn't exist. How do I make sure that Ecto passes that foreign key?
The output:
19:06:07.401 [debug] QUERY OK db=0.8ms
begin []
19:06:07.406 [debug] QUERY OK db=0.6ms
INSERT INTO "railroad_systems" ("railroad_system_name") VALUES ($1) RETURNING "id" ["test Can insert railway system"]
19:06:07.409 [debug] QUERY ERROR db=2.7ms
INSERT INTO "depos" ("capacity","depo_name","depo_uuid") VALUES ($1,$2,$3) RETURNING "id" [23, "A depo", "d387a91b-db77-4758-87ed-9951d5c2de8a"]
19:06:07.410 [debug] QUERY OK db=0.1ms
rollback []
1) test Can insert railway system (RailroadServer.DatabaseTest)
apps/railroad_server/test/railroad_server/database_test.exs:9
** (Postgrex.Error) ERROR 23502 (not_null_violation) null value in column "railroad_system_id" violates not-null constraint
table: depos
column: railroad_system_id
Failing row contains (3, null, d387a91b-db77-4758-87ed-9951d5c2de8a, A depo, 23).
stacktrace:
(ecto_sql) lib/ecto/adapters/sql.ex:621: Ecto.Adapters.SQL.raise_sql_call_error/1
(ecto) lib/ecto/repo/schema.ex:649: Ecto.Repo.Schema.apply/4
(ecto) lib/ecto/repo/schema.ex:262: anonymous fn/15 in Ecto.Repo.Schema.do_insert/4
(ecto) lib/ecto/association.ex:927: Ecto.Association.BelongsTo.on_repo_change/5
(ecto) lib/ecto/association.ex:413: Ecto.Association.on_repo_change/7
(elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
(ecto) lib/ecto/association.ex:392: Ecto.Association.on_repo_change/4
(ecto) lib/ecto/repo/schema.ex:811: Ecto.Repo.Schema.process_parents/4
(ecto) lib/ecto/repo/schema.ex:242: anonymous fn/15 in Ecto.Repo.Schema.do_insert/4
(ecto) lib/ecto/association.ex:662: Ecto.Association.Has.on_repo_change/5
(ecto) lib/ecto/association.ex:432: anonymous fn/8 in Ecto.Association.on_repo_change/7
(elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
(ecto) lib/ecto/association.ex:428: Ecto.Association.on_repo_change/7
(elixir) lib/enum.ex:1948: Enum."-reduce/3-lists^foldl/2-0-"/3
(ecto) lib/ecto/association.ex:392: Ecto.Association.on_repo_change/4
(ecto) lib/ecto/repo/schema.ex:837: Ecto.Repo.Schema.process_children/5
(ecto) lib/ecto/repo/schema.ex:914: anonymous fn/3 in Ecto.Repo.Schema.wrap_in_transaction/6
(ecto_sql) lib/ecto/adapters/sql.ex:890: anonymous fn/3 in Ecto.Adapters.SQL.checkout_or_transaction/4
(db_connection) lib/db_connection.ex:1415: DBConnection.run_transaction/4
(railroad_server) lib/railroad_server/database.ex:61: RailroadServer.Database.insert_railway_system/4
Versions:
Elixir - 1.9.0
Ecto - 3.17
Postgrex - 0.14.3
Prosgres - 11.4

Answer 1

When I try an insert with identical code as in your insert_railway_system(), I do not get a NULL column error.
My schemas are similar. The only significant difference in my code is in the changeset where I have the constraint:
|> assoc_constraint()
instead of:
|> foreign_key_constraint()
But I changed my code to try the foreign_key_constraint(), leaving the argument the same, which is equivalent to your :railroad_system, and the insert still worked. The insert also worked when I did the equivalent of foreign_key_constraint(:railroad_system_id). In fact, if I use foreign_key_constraint(:hello_world), the insert still works, so as far as I can tell, the second argument to foreign_key_constraint() is ignored, which is puzzling. I even did mix ecto.reset, which deletes the repo/database, recreates the repo/database, then executes the migrations, which creates the tables in the repo/database, and I got the same results.
My "create_depos" migration has the equivalent of the following:
add :railroad_system_id, references(:railroad_systems)
Please post:
The create_depos() function (although for me just using an attributes map instead of a changeset also worked)
The full stack trace of the error.
Your migrations.