Ecto insert_or_update creating multiple inserts? - postgresql

Question

My code is resulting in a rare double or triple insert into the database and I am at a loss as to why. It is very difficult to reproduce but I can look at the timestamps to see the created at time is basically the same when it happens. I believe it only occurs when the CardMeta is not already found.
I figure I need to add a unique key or wrap it in a transaction.
def get_or_create_meta(user, card) do
case Repo.all(from c in CardMeta, where: c.user_id == ^user.id,
where: c.card_id == ^card.id) do
[] ->
%CardMeta{}
metas ->
hd metas
end
end
def bury(user, card) do
get_or_create_meta(user, card)
|> Repo.preload([:card, :user])
|> CardMeta.changeset(%{last_seen: DateTime.utc_now(), user_id: user.id, card_id: card.id,
learning: false, known: false, prev_interval: 0})
|> Repo.insert_or_update
end
Edit: adding changeset source
def changeset(struct, params \\ %{}) do
struct
|> cast(params, [:last_seen, :difficulty, :prev_interval, :due, :known, :learning,
:user_id, :card_id])
|> assoc_constraint(:user)
|> assoc_constraint(:card)
end
Calling bury from the controller
def update(conn, %{"currentCardId" => card_id, "command" => command}) do
# perform some update on card
card = Repo.get!(Card,card_id)
user = Guardian.Plug.current_resource(conn)
case command do
"fail" ->
SpacedRepetition.fail(user, card)
"learn" ->
SpacedRepetition.learn(user, card)
_ ->
SpacedRepetition.bury(user, card)
end
sendNextCard(conn, user)
end
Edit:
I noticed the last_seen field is microseconds different between duplicated rows, whereas the create_at field does not have that resolution. Thus I suspect the insert_or_update call is fine, but the controller is firing twice before the DB updates. This could be something on the client side, which I don't want to think about. So I am just going to add a unique key.

Answer 1

As an alternative to #aliCna's answer, if you don't want to change the primary key on CardMeta, you can put a unique index constraint in the database with a migration:
defmodule YourApp.Repo.Migrations.AddCardMetaUniqueIndex do
use Ecto.Migration
def change do
create unique_index(
:card_meta,
[:card_id, :user_id],
name: :card_meta_unique_index)
end
end
Which you can then handle in your changeset to produce nice errors if conflicts occur:
def changeset(struct, params \\ %{}) do
struct
|> cast(params, [:last_seen, :difficulty, :prev_interval, :due, :known, :learning,
:user_id, :card_id])
|> assoc_constraint(:user)
|> assoc_constraint(:card)
|> unique_constraint(:user_id, name: :card_meta_unique_index)
end

Answer 2

I believe you can solve this by adding a composite primary key on user_id and card_id
defmodule Anything.CardMeta do
use Anything.Web, :model
#primary_key false
schema "card_meta" do
field :user_id, :integer, primary_key: true
field :card_id, :integer, primary_key: true
. . .
timestamps()
end
end
If this does't solve your problem please add your data model here!