Rails & SQLite: migration to add unique index and allow null values - ruby-on-rails

Question

I want to force e-mails to be unique at the database level, but I also want to allow for null e-mail values. I get the following error with the migration.
Current Schema
create_table "users", force: true do |t|
t.string "email", default: ""
...
end
add_index "users", ["email"], name: "index_users_on_email"
Attempted Migration
remove_index :users, [:email]
change_column :users, :email, :string, :null => true
add_index :users, [:email], unique: true
Error with migration
== RemoveUniqueEmailFromUsers: reverting =====================================
-- remove_index(:users, [:email])
-> 0.0437s
-- change_column(:users, :email, :string, {:null=>true})
-> 0.2664s
-- add_index(:users, [:email], {:unique=>true})
rake aborted!
An error has occurred, this and all later migrations canceled:
SQLite3::ConstraintException: indexed columns are not unique: CREATE UNIQUE INDEX "index_users_on_email" ON "users" ("email")

Answer 1

Try to remove the default: "". I think you have contradicting configs.
Empty string is not nil.

Answer 2

The problem was that the last user I created had the email of ''. After destroying this user the migration proceeded.

Answer 3

If you see the SQLite::ConstraintException after you've run $ rake db:migrate:
SQLite::ConstraintException : indexed columns are not unique CREATE UNIQUE INDEX "index_users_on_email" ON "users"
... and you're working in the dev environment, and you do not need the data in the development database, you can fix this rather quickly.
You can find out if you have existing users in your database and delete them all like this:
$ rails console
> User.count
=> 4
> User.destroy_all
> ...
> User.count
=> 0
Then you can run $ rake db:migrate again, and the migrations should run successfully.
If you're not sure, you can run $ rake db:migrate:status. If all migrations are "up," then your migrations have run successfully.
Here's how to understand the problem:
You have existing users without email values (users table has no email attribute).
When you run the migration the first part adds the email column.
Now, all of your users have an email attribute, but with a null value in the database.
The last part of the migration tries to add an index, with the condition that the values are unique (that's what unique: true specifies).
The unique constraint is impossible to add to the database because you now have an email column for the users table, which all have null values, making them non-unique values. SQLite is throwing an error, stating that you cannot add a unique constraint to a column that has non-unique values.
This can be a good thing if you're trying to add this constraint to a database column in production mode! Thanks for protecting us, SQLite!
Once you understand the problem, you can see that there are other possible solutions, but if you're in the dev environment on a new application, this will probably fit your needs.