martasd
Check a complex "unique" constraint with Ecto
In my Ecto schema, I have a User who has_many Items. An item has a boolean field active:
User module:
schema "users" do
field :name, :string
has_many :items, Item
end
Item module:
schema "items" do
field(:active, :boolean)
belongs_to(:user, User)
end
How can I ensure that a User has at most one item active? Thus, if a user already has an active item, the insertion of another active item should result in an error. The insertion of a new item with active: false should succeed, though.
It seems that exclusion_constraint could be used for that, but I haven’t found any docs showing how to use for anything other than overlapping time intervals.
Marked As Solved
LostKobrakai
A partial unique constraint can do that as well.
create unique_index(:table, [:user_id], where: "active = TRUE")
Also Liked
christhekeele
Adding onto this, you can coerce false to nil in your Item changesets to prevent accidentally writing a false.
I think this approach may work even cleaner.
Either way, after doing one of them you can add :active_item like so to your User schema for easy access:
schema "users" do
field :name, :string
has_many :items, Item
has_one :active_item, Item, where: [active: true]
timestamps()
end
wmnnd
The easiest way to solve this is probably to store nil/NULL instead of false. Then you can use a regular unique index because it only applies to non-NULL values.
martasd
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