Struct compiler now checks if a class exists before creating a new one

A class may already exist and just not be available (yet) because it hasn't been autoloaded. The StructCompiler class should call const_get in an attempt to load this class. If it can load the class, then it should use this class and not go to Dry::Core::ClassBuilder to build an entirely new class.

mapper/lib/rom/struct_compiler.rb

@@ -34,11 +34,13 @@ def call(*args)
         if attributes.empty?
           ROM::OpenStruct
         else
-          build_class(name, ROM::Struct, ns) do |klass|
-            attributes.each do |(attr_name, type)|
-              klass.attribute(attr_name, type)
-            end
+          klass = get_class(name, ns) || build_class(name, ROM::Struct, ns)
+
+          attributes.each do |(attr_name, type)|
+            klass.attribute(attr_name, type)
           end
+
+          klass
         end
       end
     end
@@ -95,6 +97,15 @@ def visit_enum(node)
       visit(type_node)
     end
 
+    # @api private
+    def get_class(name, ns)
+      begin
+        ns.const_get(class_name(name))
+      rescue NameError
+        nil
+      end
+    end
+
     # @api private
     def build_class(name, parent, ns, &block)
       Dry::Core::ClassBuilder.

mapper/spec/unit/rom/struct_compiler_spec.rb

@@ -11,6 +11,10 @@ def attr_ast(name, type, **opts)
     [:users, [[:attribute, attr_ast(:id, :Int)], [:attribute, attr_ast(:name, :String)]]]
   end
 
+  before do
+    ROM::Struct.send(:remove_const, :User) if ROM::Struct.constants.include?(:User)
+  end
+
   context 'ROM::Struct' do
     it 'generates a struct for a given relation name and columns' do
       struct = struct_compiler[*input, ROM::Struct]