Blah

lib/baby_squeel.rb

@@ -7,6 +7,14 @@
 require 'baby_squeel/active_record/query_methods'
 require 'baby_squeel/active_record/where_chain'
 
+# FIXME: This DEFINITELY doesn't belong here.
+class Arel::Nodes::TableAlias
+  def on(node = nil, &block)
+    table = BabySqueel::Table.new(self)
+    table.on(node, &block)
+  end
+end
+
 module BabySqueel
   class << self
     # Configures BabySqueel using the given block

lib/baby_squeel/dsl.rb

@@ -93,16 +93,6 @@ def quoted(value)
       sql _scope.connection.quote(value)
     end
 
-    # Evaluates a DSL block. If arity is given, this method
-    # `yield` itself, rather than `instance_eval`.
-    def evaluate(&block)
-      if block.arity.zero?
-        instance_eval(&block)
-      else
-        yield(self)
-      end
-    end
-
     private
 
     def method_missing(meth, *args, &block)

lib/baby_squeel/table.rb

@@ -51,15 +51,29 @@ def inner! # :nodoc:
     end
 
     # Specify an explicit join.
-    def on(node)
-      clone.on! node
+    def on(node = nil, &block)
+      clone.on!(node, &block)
     end
 
-    def on!(node) # :nodoc:
-      self._on = node
+    def on!(node = nil, &block) # :nodoc:
+      if node.nil? && !block_given?
+        raise 'FIXME: A proper error should be risen here!'
+      end
+
+      self._on = node || evaluate(&block)
       self
     end
 
+    # Evaluates a DSL block. If arity is given, this method
+    # `yield` itself, rather than `instance_eval`.
+    def evaluate(&block)
+      if block.arity.zero?
+        instance_eval(&block)
+      else
+        yield(self)
+      end
+    end
+
     # When referencing a joined table, the tables that
     # attributes reference can change (due to aliasing).
     # This method allows BabySqueel::Nodes::Attribute
@@ -98,7 +112,11 @@ def not_found_error!
     end
 
     def resolve(name)
-      self[name]
+      if _table.kind_of?(Arel::Nodes::TableAlias) && _table.name == name.to_s
+        self
+      else
+        self[name]
+      end
     end
 
     def respond_to_missing?(name, *)

spec/integration/joining_spec.rb

@@ -1,6 +1,23 @@
 require 'spec_helper'
 
 describe BabySqueel::ActiveRecord::QueryMethods, '#joining' do
+  context 'joining relations' do
+    it 'allows on to accept a block' do
+      authors = Author.where(id: 2)
+      relation = Post.joining do
+        authors.as('a').on { a.id == author_id }
+      end
+
+      expect(relation).to produce_sql(<<-EOSQL)
+        SELECT "posts".* FROM "posts"
+        INNER JOIN (
+          SELECT "authors".* FROM "authors"
+          WHERE "authors"."id" = 2
+        ) a ON a."id" = a."author_id"
+      EOSQL
+    end
+  end
+
   context 'when joining explicitly' do
     it 'inner joins' do
       relation = Post.joining {