Allow for defining default transformer

README.md

@@ -759,6 +759,25 @@ class UserBlueprint < Blueprinter::Base
 end
 ```
 
+#### Global Transforms
+
+You can also specify global default transformers. Create one or more transformer classes extending from `Blueprinter::Transformer` and set the `default_transformers` configuration
+```ruby
+class LowerCamelTransformer < Blueprinter::Transformer
+  def transform(hash, _object, _options)
+    hash.transform_keys! { |key| key.to_s.camelize(:lower).to_sym }
+  end
+end
+```
+
+```ruby
+Blueprinter.configure do |config|
+  config.default_transformers = [LowerCamelTransformer]
+end
+```
+
+**Note: Any transforms specified on a per-blueprint or per-view level will override the `default_transformers` in the configuration.**
+
 ---
 </details>
 

lib/blueprinter/configuration.rb

@@ -1,6 +1,6 @@
 module Blueprinter
   class Configuration
-    attr_accessor :association_default, :datetime_format, :field_default, :generator, :if, :method, :sort_fields_by, :unless, :extractor_default
+    attr_accessor :association_default, :datetime_format, :field_default, :generator, :if, :method, :sort_fields_by, :unless, :extractor_default, :default_transformers
 
     VALID_CALLABLES = %i(if unless).freeze
 
@@ -14,6 +14,7 @@ def initialize
       @sort_fields_by = :name_asc
       @unless = nil
       @extractor_default = AutoExtractor
+      @default_transformers = []
     end
 
     def jsonify(blob)

lib/blueprinter/view.rb

@@ -2,18 +2,22 @@ module Blueprinter
   # @api private
   DefinitionPlaceholder = Struct.new :name, :view?
   class View
-    attr_reader :excluded_field_names, :fields, :included_view_names, :name, :transformers, :definition_order
+    attr_reader :excluded_field_names, :fields, :included_view_names, :name, :view_transformers, :definition_order
 
-    def initialize(name, fields: {}, included_view_names: [], excluded_view_names: [],transformers: [])
+    def initialize(name, fields: {}, included_view_names: [], excluded_view_names: [], transformers: [])
       @name = name
       @fields = fields
       @included_view_names = included_view_names
       @excluded_field_names = excluded_view_names
-      @transformers =  transformers
+      @view_transformers = transformers
       @definition_order = []
       @sort_by_definition = Blueprinter.configuration.sort_fields_by.eql?(:definition)
     end
 
+    def transformers
+      view_transformers.empty? ? Blueprinter.configuration.default_transformers : view_transformers
+    end
+
     def track_definition_order(method, is_view = true)
       if @sort_by_definition
         @definition_order << DefinitionPlaceholder.new(method, is_view)
@@ -33,8 +37,8 @@ def inherit(view)
         exclude_field(field_name)
       end
 
-      view.transformers.each do |transformer|
-        self.add_transformer(transformer)
+      view.view_transformers.each do |transformer|
+        add_transformer(transformer)
       end
     end
 
@@ -61,7 +65,7 @@ def exclude_fields(field_names)
     end
 
     def add_transformer(custom_transformer)
-      transformers << custom_transformer
+      view_transformers << custom_transformer
     end
 
     def <<(field)

spec/integrations/shared/base_render_examples.rb

@@ -481,6 +481,35 @@ def transform(result_hash, object, options={})
     it('returns json with values derived from options') { should eq(result) }
   end
 
+  context 'Given blueprint has a transformer with a default configured' do
+    let(:default_transform) do
+      UpcaseKeysTransformer = Class.new(Blueprinter::Transformer) do
+        def transform(hash, _object, _options)
+          hash.transform_keys! { |key| key.to_s.upcase.to_sym }
+        end
+      end
+    end
+    before do
+      Blueprinter.configure { |config| config.default_transformers = [default_transform] }
+    end
+    after { reset_blueprinter_config! }
+    subject { blueprint.render(obj) }
+    let(:result) { '{"id":' + obj_id + ',"full_name":"Meg Ryan"}' }
+    let(:blueprint) do
+      DynamicFieldsTransformer = Class.new(Blueprinter::Transformer) do
+        def transform(result_hash, object, options={})
+          dynamic_fields = (object.is_a? Hash) ? object[:dynamic_fields] : object.dynamic_fields
+          result_hash.merge!(dynamic_fields)
+        end
+      end
+      Class.new(Blueprinter::Base) do
+        identifier :id
+        transform DynamicFieldsTransformer
+      end
+    end
+    it('overrides the configured default transformer') { should eq(result) }
+  end
+
   context "Ordering of fields from inside a view by definition" do
     before { Blueprinter.configure { |config| config.sort_fields_by = :definition } }
     after { reset_blueprinter_config! }

spec/units/configuration_spec.rb

@@ -9,6 +9,9 @@
     let(:extractor) do
       class FoodDehydrator < Blueprinter::AutoExtractor; end
     end
+    let(:transform) do
+      class UpcaseTransform < Blueprinter::Transformer; end
+    end
 
     it 'should set the `generator`' do
       Blueprinter.configure { |config| config.generator = Oj }
@@ -71,6 +74,10 @@ class FoodDehydrator < Blueprinter::AutoExtractor; end
       expect(Blueprinter.configuration.extractor_default).to eq(Blueprinter::AutoExtractor)
     end
 
+    it 'should set the `default_transformers` option' do
+      Blueprinter.configure { |config| config.default_transformers = [transform] }
+      expect(Blueprinter.configuration.default_transformers).to eq([transform])
+    end
   end
 
   describe "::Configuration" do

spec/units/view_spec.rb

@@ -32,16 +32,16 @@
     end
   end
 
-  describe '#exclude_fields(:view_name)' do 
-    it 'should return [:view_name]' do 
+  describe '#exclude_fields(:view_name)' do
+    it 'should return [:view_name]' do
       expect(view.exclude_fields([:last_name,:middle_name])).to eq([:last_name,:middle_name])
     end
-    it 'should set #excluded_field_names to [:view_name]' do 
+    it 'should set #excluded_field_names to [:view_name]' do
       view.exclude_fields([:last_name,:middle_name])
       expect(view.excluded_field_names).to eq([:last_name,:middle_name])
     end
   end
-  
+
   describe '#<<(field)' do
     context 'Given a field that does not exist' do
       it('should return field') { expect(view << field).to eq(field) }
@@ -76,6 +76,37 @@
       end
     end
   end
+
+  context 'with default transform' do
+    let(:default_transform) do
+      class DefaultTransform < Blueprinter::Transformer; end
+      DefaultTransform
+    end
+    let(:override_transform) do
+      class OverrideTransform < Blueprinter::Transformer; end
+      OverrideTransform
+    end
+    let(:view_with_default_transform) do
+      Blueprinter::View.new('View with default transform')
+    end
+    let(:view_with_override_transform) do
+      Blueprinter::View.new('View with override transform', transformers: [override_transform])
+    end
+
+    before do
+      Blueprinter.configure { |config| config.default_transformers = [default_transform] }
+    end
+
+    describe '#transformers' do
+      it 'should return the default transformers' do
+        expect(view_with_default_transform.transformers).to eq([default_transform])
+      end
+
+      it 'should allow for overriding the default transformers' do
+        expect(view_with_override_transform.transformers).to eq([override_transform])
+      end
+    end
+  end
 end
 
 class MockField