Allow explicit as well as non-existent roots

lib/hanami/db/repo.rb

@@ -5,6 +5,18 @@ module DB
     # @api public
     # @since 2.2.0
     class Repo < ROM::Repository
+      # @api public
+      # @since 2.2.0
+      def self.[](root)
+        fetch_or_store(root) do
+          # Override ROM::Repository.[] logic to ensure repos with explicit roots inherit from
+          # Hanami::DB::Repo itself, instead of the plain old ROM::Repository::Root.
+          Class.new(self).tap { |klass|
+            klass.root(root)
+          }
+        end
+      end
+
       # @api public
       # @since 2.2.0
       defines :root
@@ -24,7 +36,17 @@ def self.inherited(klass)
       def initialize(*, **)
         super
 
-        @root = set_relation(self.class.root) if self.class.root
+        # Repos in Hanami apps infer a root from their class name (e.g. :posts for PostRepo). This
+        # means _every_ repo ends up with an inferred root, many of which will not exist as
+        # relations. To avoid errors from fetching these non-existent relations, check first before
+        # setting the root.
+        @root = set_relation(self.class.root) if set_relation?(self.class.root)
+      end
+
+      private
+
+      def set_relation?(name)
+        name && container.relations.key?(name)
       end
     end
   end