Top merges things in each key one by one by calling `merge`

lib/vagrant/config/top.rb

@@ -7,6 +7,8 @@ module Vagrant
     #
     # If you're looking to create your own configuration class, see {Base}.
     class Top < Base
+      attr_reader :keys
+
       def initialize(registry=nil)
         @keys = {}
         @registry = registry || Vagrant.config_keys
@@ -28,6 +30,24 @@ module Vagrant
         end
       end
 
+      # Custom implementation to merge each key separately.
+      def merge(other)
+        result = self.class.new
+        @keys.each do |key, value|
+          result.keys[key] = value.merge(other.send(key))
+        end
+
+        other.keys.each do |key, value|
+          if !@keys.has_key?(key)
+            # This is a key that the other configuration class has
+            # that we don't, so just copy it in.
+            result.keys[key] = value.dup
+          end
+        end
+
+        result
+      end
+
       # Validates the configuration classes of this instance and raises an
       # exception if they are invalid. If you are implementing a custom configuration
       # class, the method you want to implement is {Base#validate}. This is

test/unit/vagrant/config/top_test.rb

@@ -24,4 +24,46 @@ describe Vagrant::Config::Top do
   it "still raises a method missing error if invalid key" do
     expect { instance.foo }.to raise_error(NoMethodError)
   end
+
+  describe "merging" do
+    let(:foo_class) do
+      Class.new do
+        attr_accessor :one
+        attr_accessor :two
+
+        def merge(other)
+          result = self.class.new
+          result.one = other.one || one
+          result.two = other.two || two
+          result
+        end
+      end
+    end
+
+    it "merges each key by calling `merge` on the class" do
+      registry.register(:foo, foo_class)
+
+      instance.foo.one = 1
+      instance.foo.two = 2
+
+      another = described_class.new(registry)
+      another.foo.one = 2
+
+      result = instance.merge(another)
+      result.foo.one.should == 2
+      result.foo.two.should == 2
+    end
+
+    it "merges keys that aren't in the source instance" do
+      reg = Vagrant::Registry.new
+      reg.register(:foo, foo_class)
+
+      another = described_class.new(reg)
+      another.foo.one = 2
+
+      result = instance.merge(another)
+      result.foo.one.should == 2
+      result.foo.two.should be_nil
+    end
+  end
 end