如何在Spring Data Mongo的聚合投影字段中嵌套字段

Question

When written by hand, the $project step in my aggregation pipeline looks like:

    {  
     "$project":{  
        "DRIVE":{
           "componentSummary":{"manufacturer" : "$_id.DRIVE_manufacturer"},
           "componentCount":"$_id.DRIVE_componentCount"
        },
        "hostnames":1,
        "_id":0
     }
  }

I understand that I can use the ProjectionOperationBulder to create a single level of nesting (using builder.nested ), to make something like, say:

    {  
     "$project":{  
        "DRIVE":{  
           "manufacturer":"$_id.DRIVE_manufacturer"
        },
        "hostnames":1,
        "_id":0
     }
  }

But I can't seem to figure out how to nest another level deep, as the Field interface only allows for a String name and a String target, rather than being able to define antother Field as the target.

Thanks!

Answer 1

For anyone else struggling with this -- Spring Data Mongo does not natively support multi level nesting as of this writing (stable version 1.9.5). However, as of 1.9.3, it does support custom AggregationExpressions that allow you to define the behavior yourself. Be aware that if you go down this route, you'll have to build the JSON for the query mostly by hand. My implementation is pretty quick and dirty but here it is for reference's sake.

      protected class NestedField implements Field {

private String name;
private List<Field> fields;

public NestedField(String name, List<Field> fields) {
  this.name = name;
  this.fields = fields;
}

public List<Field> getFields() {
  return fields;
}

@Override
public String getName() {
  return name;
}

private String escapeSystemVariables(String fieldTarget) {
  if (fieldTarget.startsWith("_id")) {
    return StringUtils.prependIfMissing(fieldTarget, "$");
  } else {
    return fieldTarget;
  }
}

private String encloseStringInQuotations(String quotable) {
  return JSON.serialize(quotable);
}

private String buildSingleFieldTarget(Field field) {
  if (field instanceof NestedField) {
    return String.join(":", encloseStringInQuotations(field.getName()), field.getTarget());
  }
  return String.join(":", encloseStringInQuotations(field.getName()), encloseStringInQuotations(escapeSystemVariables(
    field.getTarget())));
}

private String buildFieldTargetList(List<Field> fields) {
  List<String> fieldStrings = new ArrayList<>();
  fields.forEach(field -> {
    fieldStrings.add(buildSingleFieldTarget(field));
  });
  return Joiner.on(",").skipNulls().join(fieldStrings);
}

@Override
public String getTarget() {
  // TODO Auto-generated method stub
  return String.format("{%s}", buildFieldTargetList(fields));
}


@Override
public boolean isAliased() {
  return true;
}

}

    protected class NestedProjection implements AggregationExpression {

private List<Field> projectedFields;

public NestedProjection(List<Field> projectedFields) {
  this.projectedFields = projectedFields;

}@Override
public DBObject toDbObject(AggregationOperationContext context) {
  DBObject projectionExpression = new BasicDBObject();
  for(Field f : projectedFields) {
    //this is necessary because if we just put f.getTarget(), spring-mongo will attempt to JSON-escape the string
    DBObject target = (DBObject) com.mongodb.util.JSON.parse(f.getTarget());
      projectionExpression.put(f.getName(), target);


  }
  return projectionExpression;
}

}