Implement primitive! method

README.md

@@ -239,8 +239,20 @@ json.partial! 'sub_template', locals: { user: user }
 json.partial! 'sub_template', user: user
 ```
 
+You can explicitly make Jbuilder object return a primitive value:
 
-You can explicitly make Jbuilder object return null if you want:
+``` ruby
+json.extract! @post, :id, :title, :content, :published_at
+json.author do
+  if @post.pseudonymous?
+    json.primitive! @post.author_pseudonym
+  else
+    json.first_name @post.author_first_name
+    json.last_name @post.author_last_name
+  end
+```
+
+Returning null is also possible:
 
 ``` ruby
 json.extract! @post, :id, :title, :content, :published_at

lib/jbuilder.rb

@@ -257,6 +257,17 @@ def call(object, *attributes, &block)
     end
   end
 
+  # Returns a primitive JSON
+  # Example:
+  #
+  #   json.primitive! "hello"
+  #
+  #   "hello"
+  def primitive!(value)
+    raise PrimitiveError.build(value) unless _is_primitive?(value)
+    @attributes = value
+  end   
+
   # Returns the nil JSON.
   def nil!
     @attributes = nil
@@ -354,6 +365,10 @@ def _is_collection?(object)
     _object_respond_to?(object, :map, :count) && NON_ENUMERABLES.none?{ |klass| klass === object }
   end
 
+  def _is_primitive?(value)
+    (value.is_a? ::String) || (value.is_a? ::Numeric) || value == true || value == false || value.nil?
+  end  
+
   def _blank?(value=@attributes)
     BLANK == value
   end

lib/jbuilder/errors.rb

@@ -1,6 +1,13 @@
 require 'jbuilder/jbuilder'
 
 class Jbuilder
+  class PrimitiveError < ::StandardError
+    def self.build(value)
+      message = "Can't use #{value.inspect} as primitive"
+      new(message)
+    end
+  end
+
   class NullError < ::NoMethodError
     def self.build(key)
       message = "Failed to add #{key.to_s.inspect} property to null object"

test/jbuilder_test.rb

@@ -831,6 +831,57 @@ class JbuilderTest < ActiveSupport::TestCase
     Jbuilder.send(:class_variable_set, '@@ignore_nil', false)
   end
 
+  test 'primitive! with String value' do
+    result = jbuild do |json|
+      json.primitive! 'hello'
+    end
+
+    assert_equal 'hello', result
+  end
+
+  test 'primitive! with Numeric value' do
+    result = jbuild do |json|
+      json.primitive! 13.37
+    end
+
+    assert_equal 13.37, result
+  end
+
+  test 'primitive! with true value' do
+    result = jbuild do |json|
+      json.primitive! true
+    end
+
+    assert_equal true, result
+  end
+
+  test 'primitive! with false value' do
+    result = jbuild do |json|
+      json.primitive! false
+    end
+
+    assert_equal false, result
+  end
+
+  test 'primitive! with nil value' do
+    result = jbuild do |json|
+      json.primitive! nil
+    end
+
+    assert_equal nil, result
+  end
+
+  test 'primitive! in an empty block' do
+    result = jbuild do |json|
+      json.author do
+        json.primitive! 'David Heinemeier Hansson'
+      end
+    end
+
+    assert result.key?('author')
+    assert_equal 'David Heinemeier Hansson', result['author']
+  end  
+
   test 'nil!' do
     result = jbuild do |json|
       json.key 'value'
@@ -871,6 +922,14 @@ class JbuilderTest < ActiveSupport::TestCase
     assert !attributes.present?
   end
 
+  test 'throws PrimitiveError when trying to use hash as primitive' do
+    assert_raise Jbuilder::PrimitiveError do
+      jbuild do |json|
+        json.primitive!({})
+      end
+    end
+  end
+
   test 'throws ArrayError when trying to add a key to an array' do
     assert_raise Jbuilder::ArrayError do
       jbuild do |json|
@@ -930,6 +989,20 @@ class JbuilderTest < ActiveSupport::TestCase
     end
   end
 
+  test "throws MergeError when trying to replace non-empty block with primitive" do
+    assert_raise Jbuilder::MergeError do
+      jbuild do |json|
+        json.author do
+          json.name 'David'
+        end
+
+        json.author do
+          json.primitive! 'David Heinemeier Hansson'
+        end
+      end
+    end
+  end 
+
   if RUBY_VERSION >= "2.2.10"
     test "respects JSON encoding customizations" do
       # Active Support overrides Time#as_json for custom formatting.