Question

Using rails, I set up a HATBM model with a users table, a groups table and a groups_users for the joins (all with scaffold command).
Now I want to add a migration to add a unique index on both columns group_id and user_id for the table groups_users, in order to constraint the group_id/user_id couples to be unique.
The individual columns group_id and user_id are not null and indexes already.

Here is the line I want executed in the migration:

add_index :groups_users, [:user_id, :group_id], :unique => true

I get the following error:

-- add_index(:groups_users, [:user_id, :group_id], {:unique=>true})
rake aborted!
An error has occurred, this and all later migrations canceled:

SQLite3::ConstraintException: constraint failed: CREATE UNIQUE INDEX "index_groups_users_on_user_id_and_group_id" ON "groups_users" ("user_id", "group_id")

I use rails 3.2.11 on mac os with sqlite3 as the db.

I am sure there must be some obvious explanation, since I did not see any report of that error on the net, but I’m terribly stuck… What constraint is not satisfied here? I tried to remove the indexes on group_id and user_id, but it doesn’t change anything. If I create the bi-column index without uniqueness, then it works (but doen’t help…)

Someone’s help greatly appreciated…

Answer 1

Seems as if the UNIQUE constraint could not be fullfilled with the current data in the table. Check with GROUP BY and COUNT.