Bring back Repository#set_relation but deprecate it (fix #698)

It was part of private API used by hanami/db. This marks prepare_relation as replacement

repository/lib/rom/repository/relation_reader.rb

@@ -5,9 +5,11 @@ class Repository
     # @api private
     class RelationReader < ::Module
       module InstanceMethods
+        extend ::Dry::Core::Deprecations[:'rom-repository']
+
         private
 
-        # @api private
+        # @api public
         def prepare_relation(name, **)
           container
             .relations[name]
@@ -17,15 +19,21 @@ def prepare_relation(name, **)
             )
         end
 
+        # @api private
+        def set_relation(name) # rubocop:disable Naming/AccessorMethodName
+          prepare_relation(name)
+        end
+        deprecate :set_relation, :prepare_relation
+
         # @api private
         def relation_reader(cache, ...)
           cache_key = relation_cache_key(...)
           cache.fetch_or_store(*cache_key) { prepare_relation(...) }
         end
 
         # @api private
-        def relation_cache_key(name, **)
-          [name, auto_struct, struct_namespace]
+        def relation_cache_key(name, **kwargs)
+          [name, auto_struct, struct_namespace, kwargs]
         end
       end
 

repository/spec/integration/repository_spec.rb

@@ -423,4 +423,41 @@ class Post < OpenStruct; end
 
     expect(post).to be(post_from_user)
   end
+
+  context 'using #prepare_relation' do
+    let(:repo_class) do
+      Class.new(ROM::Repository[:users]) do
+        def prepare_relation(name, custom: 'default')
+          super.select_append { `'#{custom}'`.as(:extra_column) }
+        end
+      end
+    end
+
+    it 'is uses modified relation' do
+      expect(repo.users.to_a.map(&:extra_column)).to eql(%w[default default])
+      expect(repo.users(custom: 'custom').to_a.map(&:extra_column)).to eql(%w[custom custom])
+    end
+  end
+
+  context 'using #set_relation' do
+    let(:log_file) do
+      Tempfile.new('dry_deprecations')
+    end
+
+    before do
+      Dry::Core::Deprecations.set_logger!(log_file)
+    end
+
+    let(:repo_class) do
+      Class.new(ROM::Repository[:users]) do
+        def custom_users
+          set_relation(:users).select_append { `'modified'`.as(:modified) }
+        end
+      end
+    end
+
+    it "constructs a relation but it's deprecated" do
+      expect(repo.custom_users.to_a.map(&:modified)).to eql(%w[modified modified])
+    end
+  end
 end