module ActiveModel::Validations

expect(model.errors_on(:attribute)).to include("can't be blank")

expect(model).to have(n).errors_on(:attribute, :context => :create)
expect(model).to have(n).errors_on(:attribute)
expect(model).to have(1).error_on(:attribute)
expect(model).to have(:no).errors_on(:attribute)

@example

You can also use this to specify the content of the error messages.

a :context option to specify the validation context.
model.valid? in order to prepare the object's errors object. Accepts
Extension to enhance `to have` on AR Model instances. Calls

def errors_on(attribute, options = {})
  valid_args = [options[:context]].compact
  self.valid?(*valid_args)
  [self.errors[attribute]].flatten.compact
end