How can I return error messages from SQL and use these error messages in my rails app?

Question

I know unique: true makes a column unique in Ruby on Rails migration

add_index :table_name, :column_name, unique: true

I also know that validates:username, uniqueness: true makes a column unique too.

I wanna just to use add_index:table_name, :column_name, unique: true for making my specific column unique.

If I only use from add_index:table_name, :column_name, unique: true without validation in any model, how can I return error messages from SQL and use these error messages in my rails app?

eg
assume the email field is unique. and for making it unique, I just use add_index:table_name, :column_name, unique: true

User.create(name: "John", email: "john@gmail.com")
User.create(name: "Sofia", email: "john@gmail.com")    # it should returns an error

How can I get that error message and use it in my rails app?

Answer 1

To have uniqueness done right in Rails way you need to have validation in model and index in database.

Just add validates:name, uniqueness: true and you will be fine.

There could be still some rarely cases when you get SQL error message instead of ActiveRecord error because of race condition.

More info: https://guides.rubyonrails.org/active_record_validations.html#uniqueness

Answer 2

If you need to rescue duplicate errors raised by SQL you can use activerecord-rescue_from_duplicate gem. Using this gem allows whether use rails validation or not.

But generally speaking, adding uniqueness validation in your models doesn't guarantee uniqueness until you add a unique db index Check out this thread as well.