Unconstrained for Active Record converts exceptions raised because of underlying database constraints to appropriate ActiveModel::Errors

In your Gemfile:

gem 'unconstrained'

Only constraints that have to do with referential integrity are handled. This is intentional, as these require a roundrip to the database anyway. Constraints that can be enforced using validations in the application should be checked that way.

Currently only handles PostgreSQL.

Rails supports the dependent option on has_one / has_many associations, which can be used to a similar effect, ie:

has_many :children, dependent: :restrict_with_error

However, this has drawbacks. In the simple case, it offers no particular advantage, since it still needs a roundrip to the database, and since no locking takes place, there exists the possibility of an error in a system under heavy use.

In more complex scenarios, errors are bound to occur. For example, consider the following scenario:

parents
|
| <- dependent: :destroy
|
---children
   |
   | <- dependent: :restrict_with_error
   |
   ---grandchildren

Here ActiveRecord will never have an efficient way to proceed. It would either have to delete the children en masse, leading to database errors such as:

ActiveRecord::InvalidForeignKey: PG::ForeignKeyViolation: ERROR:
update or delete on table "children" violates foreign key constraint
"fk_rails_xxxxxxxxxx" on table "grandchildren"
DETAIL:  Key (id)=(xxxxxxxxxx) is still referenced from table "grandchildren".

Otherwise ActiveRecord would need to load and check every child record, which would be extremely inefficient. On the other hand, a relational database, with the appropriate constraints defined, would handle the operation swiftly and efficiently.

Now that Rails 4.2 supports foreign key constraints, one would simply define in the migration:

add_foreign_key :children, :parents, on_delete: :cascade
add_foreign_key :grandchildren, :children

and then the database exceptions would be handled by Unconstrained.