Update queryable interface so that it doesn't mutate

middleman-core/features/queryable.feature

@@ -29,3 +29,7 @@ Feature: Queryable Selector
     Then should support ordering by attribute descending
     Then should order by attribute ascending by default
     Then should exclude documents that do not own the attribute
+  Scenario: Passing queries around
+    Given a simple 'where' query
+    When I chain a where clause onto that query
+    Then the original query should remain unchanged

middleman-core/features/step_definitions/queryable_steps.rb

@@ -1,3 +1,15 @@
+Given /^a simple 'where' query$/ do
+  @query = Middleman::Sitemap::Queryable::Query.new({}).where(:foo => 'bar')
+end
+
+When /^I chain a where clause onto that query$/ do
+  @new_query = @query.where(:baz => 'foo')
+end
+
+Then /^the original query should remain unchanged$/ do
+  @query.should_not eql @new_query
+end
+
 Then /^should initialize with an attribute and an operator$/ do
   selector = ::Middleman::Sitemap::Queryable::Selector.new :attribute => :author, :operator => 'equal'
   :author.should == selector.attribute

middleman-core/lib/middleman-core/sitemap/queryable.rb

@@ -65,9 +65,12 @@ module Middleman
       end
 
       class Query
-        def initialize(model)
+        def initialize(model, opts={})
           @model = model
-          @where = {}
+          @where = opts[:where] || {}
+          @order_by = opts[:order_by]
+          @offset = opts[:offset]
+          @limit = opts[:limit]
         end
 
         def where(constraints_hash)
@@ -78,22 +81,30 @@ module Middleman
             selector_hash.update({ selector => value })
           end
           @where.merge! selector_hash
-          self
+          Query.new @model, opts
+        end
+
+        def opts
+          { :where => @where,
+            :order_by => @order_by,
+            :offset => @offset,
+            :limit => @limit
+          }
         end
 
         def order_by(field)
           @order_by = field.is_a?(Symbol) ? {field => :asc} : field
-          self
+          Query.new @model, opts
         end
 
         def offset(number)
           @offset = number
-          self
+          Query.new @model, opts
         end
 
         def limit(number)
           @limit = number
-          self
+          Query.new @model, opts
         end
 
         def first