# Constraint :unique =\> true **URL:** \ **Category:** Rails\ **Created:** [January 16, 2008, 10:42am UTC](https://www.ruby-forum.com/t/constraint-unique-true/126209 "2008-01-16T10:42:22Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![ke\_han](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/839c29/32.png) [@ke\_han](https://www.ruby-forum.com/u/ke_han)\ **Post date:** [January 16, 2008, 10:42am UTC](https://www.ruby-forum.com/t/constraint-unique-true/126209/1 "2008-01-16T10:42:22Z")
I have a migration called CreateUsers. in “self.up” I add the following columns, among others: ``` t.column :display_name, :string, :limit => 26, :null => ``` false, :unique =\> true t.column :email, :string, :unique =\> true I’m not sure where I came up with the idea that :unique =\> true could be used as a column constraint; but this is what I did. My DB is MySQL 5.0.x and I receive no errors when I rails runs the migration to create the table. I thought I had a table structure and behavior from ActiveRecord that would ensure a record could not be inserted with duplicate email address. I was wrong. My error was uncovered under Rails 1.2.3 and I see the same behavior on Rails 2.0.2. A friend tells me that in order to create a unique column constraint I need to add an index as such: add\_index :users, :email, :unique =\> true Wouldn’t this just give me an index of unique email address? It wouldn’t actually cause MySQL to throw an error on an insert with a duplicate email address, would it? Keep in mind, this is what I’m look for: I want an error to be returned by MySQL if a user record is inserted which contains a duplicate email address. Another friend tells me that part of the problem is MySQL. That MySQL won’t manage such a constraint for me and I should look to PostgreSQL for such behavior. This is ok. I don’t mind switching DBs if I get the behavior I’m looking for. So this leaves me with the following questions: 1 - Is “unique =\> true” a valid constraint for a column definition assuming the DB supports it? If not, why do I not receive an error during the migration? If so, shouldn’t it throw an error for such DB targets that don’t support it? btw, I have tried this against Postgresql 8.3 and I don’t see any such constraint created in my users table. It appears rails is silently ignoring the error. 2 - Is the method described above of adding an index the “rails way” to solve this problem? If so, can someone point me to a tutorial or docs on this way of thinking. thanks, Jon ---
**Author:** ![ke\_han](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/839c29/32.png) [@ke\_han](https://www.ruby-forum.com/u/ke_han)\ **Post date:** [January 16, 2008, 11:17am UTC](https://www.ruby-forum.com/t/constraint-unique-true/126209/2 "2008-01-16T11:17:04Z")
On 16 Jan 2008, at 09:41, ke han wrote: > That will do it. Mysql will enforce uniqueness. Fred > targets that don’t support it? btw, I have tried this against > Postgresql 8.3 and I don’t see any such constraint created in my users > table. It appears rails is silently ignoring the error. Looking at the source it rails is ignoring options it doesn’t recognise.