Spring-Data-MongoDB中$facet/$addFields/$function返回null解决方法

问题根因 #

出现返回字段全为null、自定义max字段未添加的核心原因有两个：

• 结果映射类型不匹配：$facet聚合的返回结果最外层是{facet别名: 管道输出文档数组}的结构，不是直接的业务实体列表，原代码直接将结果映射到User.class时，根层级不存在_id、username等业务字段，因此全部映射为null
• 实体类字段缺失：原User实体类未定义自定义计算的max字段，即使聚合管道计算出该字段，也无法完成映射
• 额外逻辑笔误：原Mongo原生语句的匹配条件是age > 18，最初代码中写为age >= 18，如果需要和原生语句逻辑完全对齐需要调整匹配条件

修复方案 #

定义和聚合输出结构完全匹配的DTO类完成结果映射，具体实现如下：

1. 调整聚合查询方法 #

public List<MaxFacet> aggregation1() {
    List<Object> args = new LinkedList<>();
    args.add("$name");
    args.add("$scores");
    // 定义$function自定义计算逻辑
    ScriptOperators.Function maxScoreFunction = Function.function(
            "function(name, scores) {let max = Math.max(...scores); return `Hello ${name}.  Your max score is ${max}.`}")
            .args(args).lang("js");
    // 构建facet管道，对齐原生语句的age>18匹配条件
    FacetOperation facetOperation = Aggregation.facet()
            .and(
                    Aggregation.match(Criteria.where("age").gt(18)),
                    Aggregation.addFields().addFieldWithValue("max", maxScoreFunction).build()
            ).as("aggregation");
    Aggregation agg = Aggregation.newAggregation(facetOperation);
    // 映射到和facet外层结构匹配的MaxFacet类
    return mongoOperations.aggregate(agg, mongoTemplate.getCollectionName(User.class), MaxFacet.class)
            .getMappedResults();
}

2. 定义facet外层结构映射类 #

public class MaxFacet {
    private List<UserOut> aggregation;

    public List<UserOut> getAggregation() {
        return aggregation;
    }

    public void setAggregation(List<UserOut> aggregation) {
        this.aggregation = aggregation;
    }
}

3. 定义facet内部文档输出类 #

@JsonPropertyOrder({"id","username","name","age","scores","max"})
public class UserOut {
    @JsonProperty(value="id")
    private String id;
    private String username;
    private String name;
    private Integer age;
    private String[] role;
    private Integer[] scores;
    private String max;

    // 各字段getter/setter
    public String getId() { return id; }
    public void setId(String id) { this.id = id; }
    public String getUsername() { return username; }
    public void setUsername(String username) { this.username = username; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public Integer getAge() { return age; }
    public void setAge(Integer age) { this.age = age; }
    public String[] getRole() { return role; }
    public void setRole(String[] role) { this.role = role; }
    public Integer[] getScores() { return scores; }
    public void setScores(Integer[] scores) { this.scores = scores; }
    public String getMax() { return max; }
    public void setMax(String max) { this.max = max; }
}

修复后返回结果 #

执行修复后的代码即可得到正确结果，结构示例如下：

[
    {
        "aggregation": [
            {
                "id": "62c5f16b39b3d635ab6bf60d",
                "username": "james34",
                "name": "james",
                "age": 34,
                "role": ["user", "specialuser"],
                "scores": [10,9,10],
                "max": "Hello james.  Your max score is 10."
            },
            {
                "id": "62c5f1b839b3d635ab6bf60e",
                "username": "elizabeth54",
                "name": "elizabeth",
                "age": 54,
                "role": ["user", "specialuser"],
                "scores": [10,10,10],
                "max": "Hello elizabeth.  Your max score is 10."
            }
        ]
    }
]

内容的提问来源于stack exchange，提问作者Breathd