Use $strLenCP with Spring Data MongoDB

问题

I have this mongodb query

db.getCollection('myCollection').aggregate(
    [{
        $project: {
            length: {
                $strLenCP: "$prefix"
            }
        }
    }, {
        $sort: {
            length: -1
        }
    }]
)

that I want to use into a spring java project but I can't manage to write the correct java code (the sort is not the issue).

I tried this

Aggregation agg = newAggregation(project().andExpression("strLenCP(prefix)").as("prefixLength"));
AggregationResults < RequestSettingsWithPrefixLength > results = mongoTemplate.aggregate(agg, RequestSettings.class, RequestSettingsWithPrefixLength.class);
List < RequestSettingsWithPrefixLength > requestSettingsList = results.getMappedResults();

but I am getting a null key in agg JSON (debug mode):

{
    "aggregate": "__collection__",
    "pipeline": [{
        {
            "$project": {
                "prefixLength": {
                    "null": ["$prefix"]
                }
            }
        }]
    }
}

I can see that my agg object has this projection operation:

expression -> strLenCP(prefix)
field -> AggregationField: AggregationField - name: prefixLength,  target: prefixLength, synthetic: true
params -> []

I am not sure this is correct but I can't find any documentation that uses strLenCP. I only found this test that uses the strLenCP projection: https://github.com/spring-projects/spring-data-mongodb/blob/dc57b66adfd60b4d69d1d349b4fcfa4ab0da95e7/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/SpelExpressionTransformerUnitTests.java#L922

Could someone help please?

Cheers

回答1:

The support for Mongo3.4 aggregation operators were added in 1.10.0.RC1. If you are okay with updating to release candidate version everything should just work fine.

Or you can try the following but you'll need to use 1.8.5 version.

 Aggregation aggregation = newAggregation(
  project().and(new AggregationExpression() {
      @Override
      public DBObject toDbObject(AggregationOperationContext aggregationOperationContext) {
          return new BasicDBObject("$strLenCP", "$prefix");
      }
  }).as("prefixLength")
 );

回答2:

You can use expression project(...).and(valueOf("fieldName").lengthCP()).as("fieldLength")

回答3:

@kairius answer isn't clear. This is how it should be done: Use StringOperators#valueOf followed by lengthCP():

TypedAggregation.project()
    .and(StringOperators.valueOf("prefix").lengthCP())
    .as("length")

来源：https://stackoverflow.com/questions/41550451/use-strlencp-with-spring-data-mongodb

标签

mongodb

spring-data

aggregation-framework