feat: preload foreign enumerable references

E.g. `User.join(:posts)`

Author: vladfaust

db_spec/pg/repository/query_spec.cr

@@ -133,6 +133,15 @@ describe "Repository(Postgres)#query" do
           post.editor.not_nil!.uuid.should eq new_user.uuid
         end
       end
+
+      context "with foreign enumerable join" do
+        user = repo.query(User.where(uuid: user.uuid).join(:authored_posts, select: '*')).first
+
+        it "preloads references" do
+          user.authored_posts.should_not be_nil
+          user.authored_posts.not_nil!.first.content.should eq "Blah-blah"
+        end
+      end
     end
   end
 end

src/atom/model/db_mapping.cr

@@ -133,6 +133,24 @@ class Atom
                 end
               {% end %}
 
+              # Preload foreign enumerable references
+              {% for type in MODEL_REFERENCES.select(&.["enumerable"]).select(&.["foreign"]) %}
+                # Check if current column is a reference marker (e.g. "_referrer")
+                # Do not check for `!types_already_set` because "_referrer" takes higher precedence
+                if column_name == "_" + {{type["name"].stringify}}
+
+                  # Skip marker column because it doesn't have any data
+                  rs.read
+                  column_indexer.value += 1
+
+                  # Read reference's attributes from further columns
+                  @{{type["name"]}} ||= {{type["true_type"]}}.new
+                  @{{type["name"]}}.not_nil!.push({{type["reference_type"]}}.new(rs, true, column_indexer))
+
+                  next
+                end
+              {% end %}
+
               # Unknown column in the result set
               raise DB::MappingException.new("#{{{@type}}}: cannot map column #{column_name} from a result set at index #{column_indexer.value}")
             end