Implement CASE Conditional Expression

5 files changed, 102 insertions, 1 deletions

diff --git a/lib/arel/nodes.rb b/lib/arel/nodes.rb
index 0e66d2dd0c..89f0f563ac 100644
--- a/lib/arel/nodes.rb
+++ b/lib/arel/nodes.rb
@@ -47,6 +47,9 @@ require 'arel/nodes/named_function'
 # windows
 require 'arel/nodes/window'
 
+# conditional expressions
+require 'arel/nodes/case'
+
 # joins
 require 'arel/nodes/full_outer_join'
 require 'arel/nodes/inner_join'
diff --git a/lib/arel/nodes/case.rb b/lib/arel/nodes/case.rb
new file mode 100644
index 0000000000..85f8851dbe
--- /dev/null
+++ b/lib/arel/nodes/case.rb
@@ -0,0 +1,57 @@
+module Arel
+  module Nodes
+    class Case < Arel::Nodes::Node
+      include Arel::OrderPredications
+      include Arel::Predications
+      include Arel::AliasPredication
+
+      attr_accessor :case, :conditions, :default
+
+      def initialize expression = nil, default = nil
+        @case = expression
+        @conditions = []
+        @default = default
+      end
+
+      def when condition, expression = nil
+        @conditions << When.new(Nodes.build_quoted(condition), expression)
+        self
+      end
+
+      def then expression
+        @conditions.last.right = Nodes.build_quoted(expression)
+        self
+      end
+
+      def else expression
+        @default = Else.new Nodes.build_quoted(expression)
+        self
+      end
+
+      def initialize_copy other
+        super
+        @case = @case.clone if @case
+        @conditions = @conditions.map { |x| x.clone }
+        @default = @default.clone if @default
+      end
+
+      def hash
+        [@case, @conditions, @default].hash
+      end
+
+      def eql? other
+        self.class == other.class &&
+          self.case == other.case &&
+          self.conditions == other.conditions &&
+          self.default == other.default
+      end
+      alias :== :eql?
+    end
+
+    class When < Binary # :nodoc:
+    end
+
+    class Else < Unary # :nodoc:
+    end
+  end
+end
diff --git a/lib/arel/predications.rb b/lib/arel/predications.rb
index 1d2b0de235..e9078e9c4b 100644
--- a/lib/arel/predications.rb
+++ b/lib/arel/predications.rb
@@ -198,6 +198,10 @@ Passing a range to `#not_in` is deprecated. Call `#not_between`, instead.
       grouping_all :lteq, others
     end
 
+    def when right
+      Nodes::Case.new(self).when quoted_node(right)
+    end
+
     private
 
     def grouping_any method_id, others, *extras
diff --git a/lib/arel/visitors/depth_first.rb b/lib/arel/visitors/depth_first.rb
index 22704dd038..2f71455580 100644
--- a/lib/arel/visitors/depth_first.rb
+++ b/lib/arel/visitors/depth_first.rb
@@ -16,6 +16,7 @@ module Arel
       def unary o
         visit o.expr
       end
+      alias :visit_Arel_Nodes_Else              :unary
       alias :visit_Arel_Nodes_Group             :unary
       alias :visit_Arel_Nodes_Grouping          :unary
       alias :visit_Arel_Nodes_Having            :unary
@@ -53,6 +54,12 @@ module Arel
         visit o.distinct
       end
 
+      def visit_Arel_Nodes_Case o
+        visit o.case
+        visit o.conditions
+        visit o.default
+      end
+
       def nary o
         o.children.each { |child| visit child}
       end
@@ -86,8 +93,9 @@ module Arel
       alias :visit_Arel_Nodes_Regexp             :binary
       alias :visit_Arel_Nodes_RightOuterJoin     :binary
       alias :visit_Arel_Nodes_TableAlias         :binary
-      alias :visit_Arel_Nodes_Values             :binary
       alias :visit_Arel_Nodes_Union              :binary
+      alias :visit_Arel_Nodes_Values             :binary
+      alias :visit_Arel_Nodes_When               :binary
 
       def visit_Arel_Nodes_StringJoin o
         visit o.left
diff --git a/lib/arel/visitors/to_sql.rb b/lib/arel/visitors/to_sql.rb
index ce1fdf80ce..598bf2d984 100644
--- a/lib/arel/visitors/to_sql.rb
+++ b/lib/arel/visitors/to_sql.rb
@@ -708,6 +708,35 @@ module Arel
         visit o.right, collector
       end
 
+      def visit_Arel_Nodes_Case o, collector
+        collector << "CASE "
+        if o.case
+          visit o.case, collector
+          collector << " "
+        end
+        o.conditions.each do |condition|
+          visit condition, collector
+          collector << " "
+        end
+        if o.default
+          visit o.default, collector
+          collector << " "
+        end
+        collector << "END"
+      end
+
+      def visit_Arel_Nodes_When o, collector
+        collector << "WHEN "
+        visit o.left, collector
+        collector << " THEN "
+        visit o.right, collector
+      end
+
+      def visit_Arel_Nodes_Else o, collector
+        collector << "ELSE "
+        visit o.expr, collector
+      end
+
       def visit_Arel_Nodes_UnqualifiedColumn o, collector
         collector << "#{quote_column_name o.name}"
         collector