Unique index allowing null values in rails

Question

I have a Message.rb that has columns A, B and C.

I added a unique index to Message like this:

add_index :messages, [:a, :b, :c], unique: true, name: 'mm_uniqueness'

however, I would like column A to be nullable without triggering the uniqueness constraint.

So:

original a=1, b=2, c=3
second   a=1, b=2, c=3 (not unique)

original a=null, b=2, c=3
second   a=null, b=2, c=3 (unique)

how to do it?

+3

Matthew berman May 30 '15 at 9:10

1 answer

kendrick.zulauf · Accepted Answer · 2015-05-30T11:20:31+0000

Try to create a partial index:

add_index :messages, [:a, :b, :c], unique: true, name: 'mm_uniqueness', where: 'a != null'

Unique index allowing null values ​​in rails