fix: relationship object serialization

__tests__/serializers/serializeResourceLinkage.test.ts

@@ -18,19 +18,6 @@ describe('`serializeResourceLinkage`', () => {
     chance = new Chance();
   });
 
-  describe('when given a resource linkage', () => {
-    it('should return the resource linkage as is', () => {
-      const resourceLinkage = {
-        type: chance.string(),
-        id: chance.string(),
-      };
-
-      expect(serializeResourceLinkage(resourceLinkage)).toEqual(
-        resourceLinkage,
-      );
-    });
-  });
-
   describe('when given an array of class instances', () => {
     it('should throw an error if some element in the array is not an object', () => {
       const classInstances = [1];

__tests__/serializers/serializeResourceObject.test.ts

@@ -122,8 +122,8 @@ describe('`serializeResourceObject`', () => {
       });
     });
 
-    describe('when the related class instance is a single object', () => {
-      it('should throw an error if the related class instance is not an object', () => {
+    describe('when the related class instance is not an object', () => {
+      it('should throw an error', () => {
         const key = chance.string();
 
         getMetadataBySymbolMocked.mockImplementation(
@@ -144,7 +144,9 @@ describe('`serializeResourceObject`', () => {
           `Failed to serialize relationship object for ${key} because the value is not an object.`,
         );
       });
+    });
 
+    describe('when the related class instance is an object', () => {
       it('should serialize the relationships object and return it', () => {
         const key = chance.string();
 
@@ -201,6 +203,49 @@ describe('`serializeResourceObject`', () => {
       });
     });
 
+    describe('when the related class instance is already a relationship object', () => {
+      it('should return without serializing the relationship object', () => {
+        const key = chance.string();
+
+        getMetadataBySymbolMocked.mockImplementation(
+          (_: object, symbol: symbol) => {
+            if (symbol === resourceSymbol) {
+              return 'type';
+            }
+
+            if (symbol === relationshipsSymbol) {
+              return [[key, '']];
+            }
+
+            return undefined;
+          },
+        );
+
+        collectMocked.mockImplementation((_: object, symbol: symbol) => {
+          if (symbol === idSymbol) {
+            return 'id';
+          }
+
+          return undefined;
+        });
+
+        const relatedClassInstance = {
+          id: 'some id',
+          type: 'some type',
+        };
+
+        const classInstance = {
+          [key]: {
+            data: relatedClassInstance,
+          },
+        };
+
+        const result = serializeResourceObject(classInstance);
+
+        expect(result.relationships).toBeUndefined();
+      });
+    });
+
     it('should return without the `relationships` field if no relationships are found', () => {
       getMetadataBySymbolMocked.mockImplementation(
         (_: object, symbol: symbol) => {

__tests__/serializers/utils/isRelationshipObject.test.ts

@@ -0,0 +1,35 @@
+import { isRelationshipObject } from '../../../src/serializers/utils/isRelationshipObject';
+import { isResourceLinkage } from '../../../src/serializers/utils/isResourceLinkage';
+
+jest.mock('../../../src/serializers/utils/isResourceLinkage');
+const isResourceLinkageMocked = jest.mocked(isResourceLinkage);
+
+describe('`isRelationshipObject`', () => {
+  it('should return false if `relationshipObjectCandidate` is `null`', () => {
+    const relationshipObjectCandidate = null;
+
+    expect(isRelationshipObject(relationshipObjectCandidate)).toBe(false);
+  });
+
+  it('should return false if `relationshipObjectCandidate` is not an object', () => {
+    const relationshipObjectCandidate = 'not an object';
+
+    expect(isRelationshipObject(relationshipObjectCandidate)).toBe(false);
+  });
+
+  it('should return false if `relationshipObjectCandidate` does not have a `data` property', () => {
+    const relationshipObjectCandidate = {};
+
+    expect(isRelationshipObject(relationshipObjectCandidate)).toBe(false);
+  });
+
+  it('should return true if `relationshipObjectCandidate.data` is a `JSONAPIResourceLinkage`', () => {
+    isResourceLinkageMocked.mockReturnValue(true);
+
+    const relationshipObjectCandidate = {
+      data: 'foo',
+    };
+
+    expect(isRelationshipObject(relationshipObjectCandidate)).toBe(true);
+  });
+});

src/serializers/serializeResourceLinkage.ts

@@ -3,16 +3,13 @@ import { resourceSymbol } from '../decorators/resource';
 import { collect } from './utils/collect';
 import { getMetadataBySymbol } from './utils/getMetadataBySymbol';
 import { isObject } from './utils/isObject';
-import { isResourceLinkage } from './utils/isResourceLinkage';
 
 import type { JSONAPIResourceLinkage } from '../types/resourceLinkage';
 
 export const serializeResourceLinkage = <I extends object>(
-  classInstance_s: I | JSONAPIResourceLinkage,
+  classInstanceCandidate_s: I | JSONAPIResourceLinkage,
 ): JSONAPIResourceLinkage => {
-  if (isResourceLinkage(classInstance_s)) {
-    return classInstance_s;
-  }
+  const classInstance_s = classInstanceCandidate_s as I;
 
   if (Array.isArray(classInstance_s)) {
     if (!classInstance_s.every(isObject)) {

src/serializers/serializeResourceObject.ts

@@ -17,6 +17,7 @@ import type {
   JSONAPIResourceObject,
   JSONObject,
 } from '../types';
+import { isRelationshipObject } from './utils/isRelationshipObject';
 
 export const serializeResourceObject = <I extends object>(
   classInstance: I,
@@ -43,6 +44,10 @@ export const serializeResourceObject = <I extends object>(
       );
     }
 
+    if (isRelationshipObject(relatedClassInstance_s)) {
+      return acc;
+    }
+
     acc[key.toString()] = {
       data: serializeResourceLinkage(relatedClassInstance_s),
     };

src/serializers/utils/index.ts

@@ -3,5 +3,6 @@ export * from './getMetadataBySymbol';
 export * from './collect';
 export * from './clean';
 export * from './isObject';
+export * from './isRelationshipObject';
 export * from './isResourceIdentifierObject';
-export * from './isResourceLinkage';
+export * from './isResourceLinkage';

src/serializers/utils/isRelationshipObject.ts

@@ -0,0 +1,23 @@
+import { isResourceLinkage } from './isResourceLinkage';
+
+import type { JSONAPIRelationshipObject } from '../../types/relationshipObject';
+
+export const isRelationshipObject = (
+  relationshipObjectCandidate: unknown,
+): relationshipObjectCandidate is JSONAPIRelationshipObject => {
+  if (relationshipObjectCandidate === null) {
+    return false;
+  }
+
+  if (typeof relationshipObjectCandidate !== 'object') {
+    return false;
+  }
+
+  if (!('data' in relationshipObjectCandidate)) {
+    return false;
+  }
+
+  const { data } = relationshipObjectCandidate;
+
+  return isResourceLinkage(data);
+};