This blog is part of our Rails 6 series. Rails 6.0.0.rc1 was recently released.

Rails 6 added slice! on ActiveModel::Errors. With this addition, it becomes quite easy to select just a few keys from errors and show or return them. Before Rails 6, we needed to convert the ActiveModel::Errors object to a hash before slicing the keys.

Let’s checkout how it works.

Rails 5.2

>> user = User.new

=> #<User id: nil, email: nil, password: nil, created_at: nil, updated_at: nil>

>> user.valid?

=> false

>> user.errors

=> #<ActiveModel::Errors:0x00007fc46700df10 @base=#<User id: nil, email: nil, password: nil, created_at: nil, updated_at: nil>, @messages={:email=>["can't be blank"], :password=>["can't be blank"]}, @details={:email=>[{:error=>:blank}], :password=>[{:error=>:blank}]}>

>> user.errors.slice!

=> Traceback (most recent call last):
        1: from (irb):16
NoMethodError (undefined method 'slice!' for #<ActiveModel::Errors:0x00007fa1f0e46eb8>)
Did you mean?  slice_when

>> errors = user.errors.to_h
>> errors.slice!(:email)

=> {:password=>["can't be blank"]}

>> errors

=> {:email=>["can't be blank"]}

Rails 6.0.0.beta2

>> user = User.new

=> #<User id: nil, email: nil, password: nil, created_at: nil, updated_at: nil>

>> user.valid?

=> false

>> user.errors

=> #<ActiveModel::Errors:0x00007fc46700df10 @base=#<User id: nil, email: nil, password: nil, created_at: nil, updated_at: nil>, @messages={:email=>["can't be blank"], :password=>["can't be blank"]}, @details={:email=>[{:error=>:blank}], :password=>[{:error=>:blank}]}>

>> user.errors.slice!(:email)

=> {:password=>["can't be blank"]}

>> user.errors

=> #<ActiveModel::Errors:0x00007fc46700df10 @base=#<User id: nil, email: nil, password: nil, created_at: nil, updated_at: nil>, @messages={:email=>["can't be blank"]}, @details={:email=>[{:error=>:blank}]}>

Here is the relevant pull request.