node.js - Mongoose:用投影填充对象的嵌套数组

Question

var CarSchema = new Schema({
    name: {type: String},
    partIds: [{type: Schema.Types.ObjectId, ref: 'Part'}],
});

var PartSchema = new Schema({
    name: {type: String},
    props: [
        { colour: {type: String}, shape: {type:String} }
    ],
});

例如

Car = {
       name: "BMW",
       partIds:[ObjectId("57baa43e152654f80aac36a6")]}

Part = {
        _id: ObjectId("57baa43e152654f80aac36a6"),
        name: "Piston",
        props: [{colour:"red", shape: "Cubical"},{colour:"green", shape: "cylindrical"}]

所以当我查询时我应该得到这样的文档:

Car = {
      name: "BMW",
      partIds: [{ 
_id:ObjectId("57baa43e152654f80aac36a6"), name:"Piston", props: [{colour:"red", shape:"cubical"}]
     }

props 数组只能包含红色元素

我想用零件数组填充汽车，使其 Prop 数组仅包含红色的对象。无论如何，有没有办法做到这一点，或者我必须采用老式的方式并循环遍历将其颜色与红色相匹配的 Prop 数组。

Answer 1

您可以通过在populate中提供select选项来做到这一点调用:

Car.findOne()
    .populate({
        path: 'partIds',
        select: { props: { $elemMatch: { colour: 'red' } }, name: 1 }
    })
    .exec(callback);

结果:

{ _id: 57c085451cd8dfcdf814f640,
    name: 'BMW',
    partIds:
     [ { _id: 57baa43e152654f80aac36a6,
         name: 'Piston',
         props: [ { colour: 'red', shape: 'Cubical' } ] } ] }

select 使用$elemMatch投影运算符仅选择红色 props 元素。