performing in memory joins

doc/TODO

@@ -1,16 +1,13 @@
 todo:
-- reorganize memory tests
-- implement joins in memory
 - result sets should be array relations
 - deal with table tests in algebra
 - cross-engine joins
 - blocks for joins
 - implement mnesia adapter
-- fix grouping
 - fix AR
 - rename externalize to derived.
 
-- result sets to attr correlation too
+- fix grouping
 - audit unit coverage of algebra
 - data objects
 - remove all explicit aliasing
@@ -92,6 +89,9 @@ done:
 - clean up block_given stuff
 - reorganize sql tests
 - recursive memory operations
+- reorganize memory tests
+- result sets to attr correlation too
+- implement joins in memory
 
 icebox:
 - #bind in Attribute and Expression should be doing a descend?

lib/arel/algebra/relations/row.rb

@@ -17,5 +17,9 @@ def slice(*attributes)
     def bind(relation)
       Row.new(relation, tuple)
     end
+
+    def combine(other, relation)
+      Row.new(relation, tuple + other.tuple)
+    end
   end
 end

lib/arel/engines/memory/relations/operations.rb

@@ -37,4 +37,25 @@ def eval
       raise NotImplementedError
     end
   end
+
+  class Alias < Compound
+    def eval
+      unoperated_rows
+    end
+  end
+
+  class Join < Relation
+    def eval
+      result = []
+      relation1.eval.each do |row1|
+        relation2.eval.each do |row2|
+          combined_row = row1.combine(row2, self)
+          if predicates.all? { |p| p.eval(combined_row) }
+            result << combined_row
+          end
+        end
+      end
+      result
+    end
+  end
 end

spec/arel/engines/memory/unit/relations/join_spec.rb

@@ -0,0 +1,32 @@
+require File.join(File.dirname(__FILE__), '..', '..', '..', '..', '..', 'spec_helper')
+
+module Arel
+  describe Join do
+    before do
+      @relation1 = Array.new([
+        [1, 'duck' ],
+        [2, 'duck' ],
+        [3, 'goose']
+      ], [:id, :name])
+      @relation2 = @relation1.alias
+      @relation3 = @relation1.alias
+    end
+
+    describe InnerJoin do
+      describe '#call' do
+        it 'combines the two tables where the predicate obtains' do
+          @relation1                                    \
+            .join(@relation2)                           \
+              .on(@relation1[:id].eq(@relation2[:id]))  \
+          .let do |relation|
+            relation.call.should == [
+              Row.new(relation, [1, 'duck',  1, 'duck' ]),
+              Row.new(relation, [2, 'duck',  2, 'duck' ]),
+              Row.new(relation, [3, 'goose', 3, 'goose'])
+            ]
+          end
+        end
+      end
+    end
+  end
+end