Allow associations to be defined with a block

README.md

@@ -186,6 +186,44 @@ Output:
 }
 ```
 
+#### Defining an association directly in the Blueprint
+
+You can also pass a block to an association:
+
+```ruby
+class ProjectBlueprint < Blueprinter::Base
+  identifier :uuid
+  field :name
+end
+
+class UserBlueprint < Blueprinter::Base
+  identifier :uuid
+
+  association :projects, blueprint: ProjectBlueprint do |user|
+    user.projects + user.company.projects
+  end
+end
+```
+
+Usage:
+
+```ruby
+puts UserBlueprint.render(user)
+```
+
+Output:
+
+```json
+{
+  "uuid": "733f0758-8f21-4719-875f-262c3ec743af",
+  "projects": [
+    {"uuid": "b426a1e6-ac41-45ab-bfef-970b9a0b4289", "name": "query-console"},
+    {"uuid": "5bd84d6c-4fd2-4e36-ae31-c137e39be542", "name": "blueprinter"},
+    {"uuid": "785f5cd4-7d8d-4779-a6dd-ec5eab440eff", "name": "uncontrollable"}
+  ]
+}
+```
+
 ### Passing additional properties to `render`
 
 `render` takes an options hash which you can pass additional properties, allowing you to utilize those additional properties in the `field` block. For example:

lib/blueprinter/base.rb

@@ -37,7 +37,7 @@ class Base
     #   end
     #
     # @return [Field] A Field object
-    def self.identifier(method, name: method, extractor: AutoExtractor)
+    def self.identifier(method, name: method, extractor: AutoExtractor.new)
       view_collection[:identifier] << Field.new(method, name, extractor, self)
     end
 
@@ -97,9 +97,9 @@ def self.inherited(subclass)
     # @return [Field] A Field object
     def self.field(method, options = {}, &block)
       options = if block_given?
-        {name: method, extractor: BlockExtractor, block: {method => block}}
+        {name: method, extractor: BlockExtractor.new, block: block}
       else
-        {name: method, extractor: AutoExtractor}
+        {name: method, extractor: AutoExtractor.new}
       end.merge(options)
       current_view << Field.new(method,
                                 options[:name],
@@ -119,6 +119,8 @@ def self.field(method, options = {}, &block)
     #   JSON output.
     # @option options [Symbol] :view Specify the view to use or fall back to
     #   to the :default view.
+    # @yield [Object] The object passed to `render` is also passed to the
+    #   block.
     #
     # @example Specifying an association
     #   class UserBlueprint < Blueprinter::Base
@@ -127,15 +129,29 @@ def self.field(method, options = {}, &block)
     #     # code
     #   end
     #
+    # @example Passing a block to be evaluated as the value.
+    #   class UserBlueprint < Blueprinter::Base
+    #     association :vehicles, blueprint: VehiclesBlueprint do |user|
+    #       user.vehicles + user.company.vehicles
+    #     end
+    #   end
+    #
     # @return [Field] A Field object
-    def self.association(method, options = {})
+    def self.association(method, options = {}, &block)
       raise BlueprinterError, 'blueprint required' unless options[:blueprint]
       name = options.delete(:name) || method
+
+      options = if block_given?
+        options.merge(extractor: BlockExtractor.new, block: block)
+      else
+        options.merge(extractor: AutoExtractor.new)
+      end
+
       current_view << Field.new(method,
-                                       name,
-                                       AssociationExtractor,
-                                       self,
-                                       options.merge(association: true))
+                                name,
+                                AssociationExtractor.new,
+                                self,
+                                options.merge(association: true))
     end
 
     # Generates a JSON formatted String.
@@ -220,7 +236,7 @@ def self.prepare(object, view_name:, local_options:)
     # @return [Array<Symbol>] an array of field names
     def self.fields(*field_names)
       field_names.each do |field_name|
-        current_view << Field.new(field_name, field_name, AutoExtractor, self)
+        current_view << Field.new(field_name, field_name, AutoExtractor.new, self)
       end
     end
 

lib/blueprinter/extractor.rb

@@ -1,9 +1,6 @@
 # @api private
 module Blueprinter
   class Extractor
-    def initialize
-    end
-
     def extract(field_name, object, local_options, options={})
       fail NotImplementedError, "An Extractor must implement #extract"
     end

lib/blueprinter/extractors/association_extractor.rb

@@ -1,8 +1,8 @@
 module Blueprinter
   class AssociationExtractor < Extractor
     def extract(association_name, object, local_options, options={})
-      value = object.public_send(association_name)
-      return (value || options[:default]) if value.nil?
+      value = options[:extractor].extract(association_name, object, local_options, options)
+      return options[:default] if value.nil?
       view = options[:view] || :default
       options[:blueprint].prepare(value, view_name: view, local_options: local_options)
     end

lib/blueprinter/extractors/auto_extractor.rb

@@ -1,7 +1,12 @@
 module Blueprinter
   class AutoExtractor < Extractor
+    def initialize
+      @hash_extractor = HashExtractor.new
+      @public_send_extractor = PublicSendExtractor.new
+    end
+
     def extract(field_name, object, local_options, options = {})
-      extractor = object.is_a?(Hash) ? HashExtractor : PublicSendExtractor
+      extractor = object.is_a?(Hash) ? @hash_extractor : @public_send_extractor
       extraction = extractor.extract(field_name, object, local_options, options)
       options.key?(:datetime_format) ? format_datetime(extraction, options[:datetime_format]) : extraction
     end

lib/blueprinter/extractors/block_extractor.rb

@@ -1,7 +1,7 @@
 module Blueprinter
   class BlockExtractor < Extractor
     def extract(field_name, object, local_options, options = {})
-      options[:block][field_name].call(object, local_options)
+      options[:block].call(object, local_options)
     end
   end
 end

spec/integrations/base_spec.rb

@@ -84,6 +84,22 @@
           end
           it('returns json with association') { should eq(result) }
         end
+        context 'Given block is passed' do
+          let(:blueprint) do
+            vehicle_blueprint = Class.new(Blueprinter::Base) do
+              fields :make
+            end
+
+            Class.new(Blueprinter::Base) do
+              identifier :id
+              association(:automobiles, blueprint: vehicle_blueprint) { |o| o.vehicles }
+            end
+          end
+          let(:result) do
+            '{"id":' + obj_id + ',"automobiles":[{"make":"Super Car"}]}'
+          end
+          it('returns json with aliased association') { should eq(result) }
+        end
         context 'Given no associated blueprint is given' do
           let(:blueprint) do
             Class.new(Blueprinter::Base) do