Add cross_product shortcut

  * Sugar over join with empty on-list
  * Checks that there are no duplicate attributes between the relations

lib/bmg/algebra/shortcuts.rb

@@ -57,6 +57,15 @@ def left_join(right, on = [], *args)
         self.left_join(right.rename(renaming), on.keys, *args)
       end
 
+      def cross_product(right)
+        duplicates = self.type.attrlist & right.type.attrlist
+        unless duplicates.empty?
+          raise TypeError, "Cross product incompatible - duplicate attribute(s): #{duplicates.join(', ')}"
+        end
+
+        return join(right)
+      end
+
       def matching(right, on = [])
         return super unless on.is_a?(Hash)
         renaming = Hash[on.map{|k,v| [v,k] }]

spec/unit/algebra/shortcuts/test_cross_product.rb

@@ -0,0 +1,56 @@
+require 'spec_helper'
+module Bmg
+  module Algebra
+    describe Shortcuts, "cross_product" do
+
+      let(:left) {
+        Relation.new([
+          { a: "foo",  xyz: 1 },
+          { a: "bar",  xyz: 2 }
+        ], Type::ANY.with_attrlist([:xyz, :b]))
+      }
+
+      let(:right) {
+        Relation.new([
+          { c: "baz",  d: 3 },
+          { c: "zap",  d: 4 }
+        ], Type::ANY.with_attrlist([:c, :d]))
+      }
+
+      subject {
+        left.cross_product(right)
+      }
+
+      it 'compiles as expected' do
+        expect(subject).to be_a(Operator::Join)
+        expect(left_operand(subject)).to be(left)
+        expect(right_operand(subject)).to be(right)
+        expect(subject.send(:on)).to eql([])
+      end
+
+      it 'works as expected' do
+        expect(subject.to_set).to eql([
+          { a: "bar", xyz: 2, c: "zap", d: 4 },
+          { a: "foo", xyz: 1, c: "baz", d: 3 },
+          { a: "foo", xyz: 1, c: "zap", d: 4 },
+          { a: "bar", xyz: 2, c: "baz", d: 3 },
+        ].to_set)
+      end
+
+      context 'with duplicate attributes' do
+        let(:right) {
+          Relation.new([
+            { xyz: "baz",  d: 3 },
+            { xyz: "zap",  d: 4 }
+          ], Type::ANY.with_attrlist([:xyz, :d]))
+        }
+
+        it 'raises an error' do
+          expect{
+            left.cross_product(right)
+          }.to raise_error(Bmg::TypeError, /Cross product incompatible.*xyz/)
+        end
+      end
+    end
+  end
+end