GraphQL循环引用的问题

Posted 2020-12-03 time-is-life

下面的代码中, 由于friends字段引用了PersonType字段,而friends本身又是PersonType的一部分,在运行的时候会报错:

Expected undefined to be a GraphQL type

 

var PersonType = new GraphQLObjectType({     name: ‘Person‘,     description: ‘...‘,     fields: {                 id: {                     type: GraphQLString,                     resolve : function (person) {                         return person.first_name;                     }                 },                 firstName: {                     type: GraphQLString,                     resolve : function (person) {                         return person.first_name;                     }                 },                 lastName: {                     type: GraphQLString,                     resolve : function (person) {                         return person.last_name;                     }                 },                 department: {                     type: GraphQLString,                     resolve : function (person) {                         return person.department;                     }                 },                 //email: { type: GraphQLString },                 //userName: { type: GraphQLString },                 //id: { type: GraphQLString },                 friends:  {                      type: GraphQLList(PersonType)                     //resolve: function (person) {                     //    //return person.friends.map(getPersonByUrl);                     //    return person.friends;                     //}                     }             } });

 

 

原因在于GraphQLList初始化的时候会检查PersonType的类型,而此时PersonType的定义尚未完成,所以还是undefined, 所以会报上面的错误.

[解决方案]

搜索到了这篇文章: https://gist.github.com/fbaiodias/77406c29ddf37fe46c3c

Fix Using a function to return the fields on author.js does the trick: On author.js @@ -13 +13 @@ -   fields: { +   fields: () => ({

 

把代码改成下面的就可以了.

var PersonType = new GraphQLObjectType( {     name: ‘Person‘,     description: ‘...‘,     fields: ()=>({         id: {             type: GraphQLString,             resolve : function (person) {                 return person.first_name;             }         },         firstName: {             type: GraphQLString,             resolve : function (person) {                 return person.first_name;             }         },         lastName: {             type: GraphQLString,             resolve : function (person) {                 return person.last_name;             }         },         department: {             type: GraphQLString,             resolve : function (person) {                 return person.department;             }         },         //email: { type: GraphQLString },         //userName: { type: GraphQLString },         //id: { type: GraphQLString },         friends: {             type: GraphQLList(PersonType)                     //resolve: function (person) {                     //    //return person.friends.map(getPersonByUrl);                     //    return person.friends;                     //}         }     }) });

 

原理就是把fields放到函数中后,会在另一个线程中执行,所以执行的时候PersonType已经创建完成,所以就不会报错了.