Validate multiple contexts on `valid?` and `invalid?` at once.

Example:

```ruby
class Person
  include ActiveModel::Validations

  attr_reader :name, :title
  validates_presence_of :name, on: :create
  validates_presence_of :title, on: :update
end

person = Person.new
person.valid?([:create, :update])    # => true
person.errors.messages               # => {:name=>["can't be blank"], :title=>["can't be blank"]}
```

3 files changed, 38 insertions, 1 deletions

diff --git a/activemodel/CHANGELOG.md b/activemodel/CHANGELOG.md
index dddfd940bb..625dfa404b 100644
--- a/activemodel/CHANGELOG.md
+++ b/activemodel/CHANGELOG.md
@@ -1,3 +1,21 @@
+*   Validate multiple contexts on `valid?` and `invalid?` at once.
+
+    Example:
+    
+        class Person
+          include ActiveModel::Validations
+    
+          attr_reader :name, :title
+          validates_presence_of :name, on: :create
+          validates_presence_of :title, on: :update
+        end
+    
+        person = Person.new
+        person.valid?([:create, :update])    # => true
+        person.errors.messages               # => {:name=>["can't be blank"], :title=>["can't be blank"]}
+
+    *Dmitry Polushkin*
+
 *   Ensure `method_missing` is called for methods passed to
     `ActiveModel::Serialization#serializable_hash` that don't exist.
 
diff --git a/activemodel/lib/active_model/validations.rb b/activemodel/lib/active_model/validations.rb
index 5f1dde4aa3..f23c920d87 100644
--- a/activemodel/lib/active_model/validations.rb
+++ b/activemodel/lib/active_model/validations.rb
@@ -162,7 +162,7 @@ module ActiveModel
           options = options.dup
           options[:if] = Array(options[:if])
           options[:if].unshift ->(o) {
-            Array(options[:on]).include?(o.validation_context)
+            !(Array(options[:on]) & Array(o.validation_context)).empty?
           }
         end
 
diff --git a/activemodel/test/cases/validations/validations_context_test.rb b/activemodel/test/cases/validations/validations_context_test.rb
index 150dce379f..b901a1523e 100644
--- a/activemodel/test/cases/validations/validations_context_test.rb
+++ b/activemodel/test/cases/validations/validations_context_test.rb
@@ -8,6 +8,7 @@ class ValidationsContextTest < ActiveModel::TestCase
   end
 
   ERROR_MESSAGE = "Validation error from validator"
+  ANOTHER_ERROR_MESSAGE = "Another validation error from validator"
 
   class ValidatorThatAddsErrors < ActiveModel::Validator
     def validate(record)
@@ -15,6 +16,12 @@ class ValidationsContextTest < ActiveModel::TestCase
     end
   end
 
+  class AnotherValidatorThatAddsErrors < ActiveModel::Validator
+    def validate(record)
+      record.errors[:base] << ANOTHER_ERROR_MESSAGE
+    end
+  end
+
   test "with a class that adds errors on create and validating a new model with no arguments" do
     Topic.validates_with(ValidatorThatAddsErrors, on: :create)
     topic = Topic.new
@@ -46,4 +53,16 @@ class ValidationsContextTest < ActiveModel::TestCase
     assert topic.invalid?(:context2), "Validation did not run on context2 when 'on' is set to context1 and context2"
     assert topic.errors[:base].include?(ERROR_MESSAGE)
   end
+
+  test "with a class that validating a model for a multiple contexts" do
+    Topic.validates_with(ValidatorThatAddsErrors, on: :context1)
+    Topic.validates_with(AnotherValidatorThatAddsErrors, on: :context2)
+
+    topic = Topic.new
+    assert topic.valid?, "Validation ran with no context given when 'on' is set to context1 and context2"
+
+    assert topic.invalid?([:context1, :context2]), "Validation did not run on context1 when 'on' is set to context1 and context2"
+    assert topic.errors[:base].include?(ERROR_MESSAGE)
+    assert topic.errors[:base].include?(ANOTHER_ERROR_MESSAGE)
+  end
 end