Add *_pair methods to create only two records

This introduces a set of methods for each build strategy where only two
records are created. Because the *_list methods can create an arbitrary
number (often too high), this introduces *_pair to ensure only two
records are created (and the number 2 doesn't need to be specified in
the call).

GETTING_STARTED.md

@@ -868,6 +868,13 @@ To set the attributes for each of the factories, you can pass in a hash as you n
 twenty_year_olds = FactoryGirl.build_list(:user, 25, date_of_birth: 20.years.ago)
 ```
 
+There's also a set of `*_pair` methods for creating two records at a time:
+
+```ruby
+built_users   = FactoryGirl.build_pair(:user) # array of two built users
+created_users = FactoryGirl.create_pair(:user) # array of two created users
+```
+
 Custom Construction
 -------------------
 

lib/factory_girl/strategy_syntax_method_registrar.rb

@@ -8,6 +8,7 @@ module FactoryGirl
     def define_strategy_methods
       define_singular_strategy_method
       define_list_strategy_method
+      define_pair_strategy_method
     end
 
     private
@@ -28,6 +29,14 @@ module FactoryGirl
       end
     end
 
+    def define_pair_strategy_method
+      strategy_name = @strategy_name
+
+      define_syntax_method("#{strategy_name}_pair") do |name, *traits_and_overrides, &block|
+        2.times.map { send(strategy_name, name, *traits_and_overrides, &block) }
+      end
+    end
+
     def define_syntax_method(name, &block)
       FactoryGirl::Syntax::Methods.module_exec do
         define_method(name, &block)

spec/acceptance/create_pair_spec.rb

@@ -0,0 +1,32 @@
+require 'spec_helper'
+
+describe "create multiple instances" do
+  before do
+    define_model('Post', title: :string, position: :integer)
+
+    FactoryGirl.define do
+      factory(:post) do |post|
+        post.title "Through the Looking Glass"
+        post.position { rand(10**4) }
+      end
+    end
+  end
+
+  context "without default attributes" do
+    subject { FactoryGirl.create_pair(:post) }
+
+    its(:length) { should eq 2 }
+
+    it "creates all the posts" do
+      subject.each do |record|
+        expect(record).not_to be_new_record
+      end
+    end
+
+    it "uses the default factory values" do
+      subject.each do |record|
+        expect(record.title).to eq "Through the Looking Glass"
+      end
+    end
+  end
+end

spec/acceptance/register_strategies_spec.rb

@@ -49,6 +49,14 @@ describe "register custom strategies" do
     expect(inserted_items.length).to eq 2
     expect(inserted_items.map(&:name)).to eq ["Custom strategy", "Custom strategy"]
   end
+
+  it "allows using the *_pair method to build a list using a custom strategy" do
+    FactoryGirl.register_strategy(:insert, custom_strategy)
+
+    inserted_items = FactoryGirl.insert_pair(:named_object)
+    expect(inserted_items.length).to eq 2
+    expect(inserted_items.map(&:name)).to eq ["Custom strategy", "Custom strategy"]
+  end
 end
 
 describe "including FactoryGirl::Syntax::Methods when custom strategies have been declared" do