Add relationships to resource serializers

app/json_schemas/customer_schema.rb

@@ -19,4 +19,8 @@ class CustomerSchema < JsonApiSchema
   def self.required_attributes
     [:enterprise_id, :email]
   end
+
+  def self.relationships
+    [:enterprise]
+  end
 end

app/json_schemas/json_api_schema.rb

@@ -10,6 +10,10 @@ class JsonApiSchema
       []
     end
 
+    def relationships
+      []
+    end
+
     def all_attributes
       attributes.keys
     end
@@ -83,7 +87,12 @@ class JsonApiSchema
           properties: attributes,
           required: required
         },
-        relationships: { type: :object }
+        relationships: {
+          type: :object,
+          properties: relationships.to_h do |name|
+            [name, { "$ref" => "#/components/schemas/relationship" }]
+          end
+        }
       }
     end
   end

app/json_schemas/relationship_schema.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class RelationshipSchema
+  def self.schema
+    {
+      type: :object,
+      properties: {
+        data: {
+          type: [:object, :array]
+        },
+        links: {
+          type: :object,
+          properties: {
+            related: { type: :string }
+          }
+        }
+      }
+    }
+  end
+end