1 files changed, 111 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/statement_cache.rb b/activerecord/lib/active_record/statement_cache.rb
new file mode 100644
index 0000000000..192a19f05d
--- /dev/null
+++ b/activerecord/lib/active_record/statement_cache.rb
@@ -0,0 +1,111 @@
+module ActiveRecord
+
+  # Statement cache is used to cache a single statement in order to avoid creating the AST again.
+  # Initializing the cache is done by passing the statement in the create block:
+  #
+  #   cache = StatementCache.create(Book.connection) do |params|
+  #     Book.where(name: "my book").where("author_id > 3")
+  #   end
+  #
+  # The cached statement is executed by using the +execute+ method:
+  #
+  #   cache.execute([], Book, Book.connection)
+  #
+  # The relation returned by the block is cached, and for each +execute+ call the cached relation gets duped.
+  # Database is queried when +to_a+ is called on the relation.
+  #
+  # If you want to cache the statement without the values you can use the +bind+ method of the
+  # block parameter.
+  #
+  #   cache = StatementCache.create(Book.connection) do |params|
+  #     Book.where(name: params.bind)
+  #   end
+  #
+  # And pass the bind values as the first argument of +execute+ call.
+  #
+  #   cache.execute(["my book"], Book, Book.connection)
+  class StatementCache # :nodoc:
+    class Substitute; end # :nodoc:
+
+    class Query # :nodoc:
+      def initialize(sql)
+        @sql = sql
+      end
+
+      def sql_for(binds, connection)
+        @sql
+      end
+    end
+
+    class PartialQuery < Query # :nodoc:
+      def initialize values
+        @values = values
+        @indexes = values.each_with_index.find_all { |thing,i|
+          Arel::Nodes::BindParam === thing
+        }.map(&:last)
+      end
+
+      def sql_for(binds, connection)
+        val = @values.dup
+        binds = binds.dup
+        @indexes.each { |i| val[i] = connection.quote(*binds.shift.reverse) }
+        val.join
+      end
+    end
+
+    def self.query(visitor, ast)
+      Query.new visitor.accept(ast, Arel::Collectors::SQLString.new).value
+    end
+
+    def self.partial_query(visitor, ast, collector)
+      collected = visitor.accept(ast, collector).value
+      PartialQuery.new collected
+    end
+
+    class Params # :nodoc:
+      def bind; Substitute.new; end
+    end
+
+    class BindMap # :nodoc:
+      def initialize(bind_values)
+        @indexes   = []
+        @bind_values = bind_values
+
+        bind_values.each_with_index do |(_, value), i|
+          if Substitute === value
+            @indexes << i
+          end
+        end
+      end
+
+      def bind(values)
+        bvs = @bind_values.map(&:dup)
+        @indexes.each_with_index { |offset,i| bvs[offset][1] = values[i] }
+        bvs
+      end
+    end
+
+    attr_reader :bind_map, :query_builder
+
+    def self.create(connection, block = Proc.new)
+      relation      = block.call Params.new
+      bind_map      = BindMap.new relation.bind_values
+      query_builder = connection.cacheable_query relation.arel
+      new query_builder, bind_map
+    end
+
+    def initialize(query_builder, bind_map)
+      @query_builder = query_builder
+      @bind_map      = bind_map
+    end
+
+    def execute(params, klass, connection)
+      bind_values = bind_map.bind params
+
+      sql = query_builder.sql_for bind_values, connection
+
+      klass.find_by_sql sql, bind_values
+    end
+    alias :call :execute
+  end
+end