Add a collector to grab the bind values off the AST

Now that the bind values are being stored on the actual AST, we need a
way to pull them off into the array that we were previously maintaining
separately. This requires a full walk of the AST. This is an expensive
operation, so I've also added a visitor for delegating to more than one
visitor in a single pass.

2 files changed, 56 insertions, 0 deletions

diff --git a/lib/arel/collectors/bind.rb b/lib/arel/collectors/bind.rb
new file mode 100644
index 0000000000..d816aed90d
--- /dev/null
+++ b/lib/arel/collectors/bind.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+module Arel
+  module Collectors
+    class Bind
+      def initialize
+        @binds = []
+      end
+
+      def << str
+        self
+      end
+
+      def add_bind bind
+        @binds << bind
+        self
+      end
+
+      def value
+        @binds
+      end
+    end
+  end
+end
diff --git a/lib/arel/collectors/composite.rb b/lib/arel/collectors/composite.rb
new file mode 100644
index 0000000000..4f6156fe27
--- /dev/null
+++ b/lib/arel/collectors/composite.rb
@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+module Arel
+  module Collectors
+    class Composite
+      def initialize(left, right)
+        @left = left
+        @right = right
+      end
+
+      def << str
+        left << str
+        right << str
+        self
+      end
+
+      def add_bind bind, &block
+        left.add_bind bind, &block
+        right.add_bind bind, &block
+        self
+      end
+
+      def value
+        [left.value, right.value]
+      end
+
+      protected
+
+      attr_reader :left, :right
+    end
+  end
+end