Optimise the repository relations reader

repository/lib/rom/repository/relation_reader.rb

@@ -4,27 +4,50 @@ module ROM
   class Repository
     # @api private
     class RelationReader < Module
+      extend Dry::Core::ClassAttributes
+
       # @api private
       attr_reader :klass
 
       # @api private
       attr_reader :relations
 
+      defines :relation_readers
+
+      defines :mutex
+      mutex(Mutex.new)
+
+      defines :relation_cache
+      relation_cache(Concurrent::Hash.new)
+
       module InstanceMethods
         # @api private
         def set_relation(name)
           container
             .relations[name]
-            .with(auto_struct: auto_struct)
-            .struct_namespace(struct_namespace)
+            .with(auto_struct: auto_struct, struct_namespace: struct_namespace)
+        end
+
+        def relation_reader(name, relation_cache)
+          key = [name, auto_struct, struct_namespace]
+          relation_cache[key] ||= set_relation(name)
         end
       end
 
+      # @api private
+      def mutex
+        ROM::Repository::RelationReader.mutex
+      end
+
       # @api private
       def initialize(klass, relations)
-        @klass = klass
         @relations = relations
-        define_readers!
+        mutex.synchronize do
+          unless self.class.relation_readers
+            self.class.relation_readers(build_relation_readers(relations, self.class.relation_cache))
+          end
+        end
+        klass.include self.class.relation_readers
       end
 
       # @api private
@@ -33,13 +56,16 @@ def included(klass)
         klass.include(InstanceMethods)
       end
 
+
       private
 
       # @api private
-      def define_readers!
-        relations.each do |name|
-          define_method(name) do
-            @relations[name] ||= set_relation(name)
+      def build_relation_readers(relations, relation_cache)
+        Module.new do
+          relations.each do |name|
+            define_method(name) do
+              relation_reader(name, relation_cache)
+            end
           end
         end
       end