support relations created with a table alias

Gemfile

@@ -5,7 +5,7 @@ gemspec
 if ENV['AREL']
   gem 'arel', path: ENV['AREL']
 else
-  gem 'arel'
+  gem 'arel', github: 'rails/arel'
 end
 
 gem 'mocha', '>= 0.11.2', :require => false

activerecord/lib/active_record/relation/query_methods.rb

@@ -552,7 +552,7 @@ module ActiveRecord
     end
 
     def build_arel
-      arel = table.from table
+      arel = Arel::SelectManager.new(table.engine, table)
 
       build_joins(arel, joins_values) unless joins_values.empty?
 

activerecord/test/cases/relations_test.rb

@@ -1332,4 +1332,14 @@ class RelationTest < ActiveRecord::TestCase
       assert_equal expected, relation.inspect
     end
   end
+
+  test 'using a custom table affects the wheres' do
+    table_alias = Post.arel_table.alias('omg_posts')
+
+    relation = ActiveRecord::Relation.new Post, table_alias
+    relation.where!(:foo => "bar")
+
+    node = relation.arel.constraints.first.grep(Arel::Attributes::Attribute).first
+    assert_equal table_alias, node.relation
+  end
 end