GraphQL循环引用的问题

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.