Change the deprecation messages to show the preferred way to work with

ActiveModel::Errors

activemodel/README.rdoc

@@ -49,7 +49,7 @@ behavior out of the box:
         send("#{attr}=", nil)
       end
     end
-  
+
     person = Person.new
     person.clear_name
     person.clear_age
@@ -132,7 +132,7 @@ behavior out of the box:
         "Name"
       end
     end
-  
+
     person = Person.new
     person.name = nil
     person.validate!
@@ -216,10 +216,10 @@ behavior out of the box:
   {Learn more}[link:classes/ActiveModel/Validations.html]
 
 * Custom validators
-  
+
     class HasNameValidator < ActiveModel::Validator
       def validate(record)
-        record.errors.messages[:name] << "must exist" if record.name.blank?
+        record.errors.add(:name, "must exist") if record.name.blank?
       end
     end
 

activemodel/lib/active_model/errors.rb

@@ -114,9 +114,9 @@ module ActiveModel
     #   person.errors.get(:age)  # => []
     def get(key)
       ActiveSupport::Deprecation.warn(<<-MESSAGE.squish)
-        ActiveModel::Errors#get is deprecated and will be removed in Rails 5.1
+        ActiveModel::Errors#get is deprecated and will be removed in Rails 5.1.
 
-        To achieve the same use messages[:#{key}]
+        To achieve the same use model.errors[:#{key}].
       MESSAGE
 
       messages[key]
@@ -129,9 +129,9 @@ module ActiveModel
     #   person.errors.get(:name) # => ["can't be nil"]
     def set(key, value)
       ActiveSupport::Deprecation.warn(<<-MESSAGE.squish)
-        ActiveModel::Errors#set is deprecated and will be removed in Rails 5.1
+        ActiveModel::Errors#set is deprecated and will be removed in Rails 5.1.
 
-        To achieve the same use messages[:#{key}] = "#{value}"
+        Use model.errors.add(:#{key}, #{value.inspect}) instead.
       MESSAGE
 
       messages[key] = value
@@ -162,9 +162,9 @@ module ActiveModel
     #   person.errors[:name] # => ['must be set']
     def []=(attribute, error)
       ActiveSupport::Deprecation.warn(<<-MESSAGE.squish)
-        ActiveModel::Errors#[]= is deprecated and will be removed in Rails 5.1
+        ActiveModel::Errors#[]= is deprecated and will be removed in Rails 5.1.
 
-        To achieve the same use messages[:#{attribute}] << "#{error}"
+        Use model.errors.add(:#{attribute}, #{error.inspect}) instead.
       MESSAGE
 
       messages[attribute.to_sym] << error

activemodel/lib/active_model/validator.rb

@@ -15,7 +15,7 @@ module ActiveModel
   #   class MyValidator < ActiveModel::Validator
   #     def validate(record)
   #       if some_complex_logic
-  #         record.errors.messages[:base] << "This record is invalid"
+  #         record.errors.add(:base, "This record is invalid")
   #       end
   #     end
   #

guides/source/active_record_validations.md

@@ -1078,7 +1078,7 @@ Another way to do this is using `[]=` setter
 ```ruby
   class Person < ActiveRecord::Base
     def a_method_used_for_validation_purposes
-      errors.messages[:name] << "cannot contain the characters !@#%*()_-+="
+      errors.messages.add(:name, "cannot contain the characters !@#%*()_-+=")
     end
   end