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问题描述

现阶段我的聚合管道中有两个文档:

  {
_id: Piers Morgan,
entities:[ Sexism, Charlotte Hawkins, Red地毯]
}
{
_id: Gareth Bale ,
entities:[ Sergio Busquets, Real Madrid CF, EFL Cup, Copa del Rey]
}
  {$ project:{count:{$ size: $ entities}}} 

在Morphia中,我尝试过:

  .project(projection( count,
Projection.expression( $ size, entities))));

返回java.lang.ClassCastException:java.lang.String无法转换为com.mongodb。 DBObject



Morphia中正确的等效表达式是什么?

解决方案

您应该使用 projection 而不是 expression 这样的

  .project(projection( count,projection( $ size, entities)))


I have two documents at this stage in my aggregation pipeline which are:

{
    "_id" : "Piers Morgan",
    "entities" : ["Sexism", "Charlotte Hawkins","Red carpet"]
}
{
    "_id" : "Gareth Bale",
    "entities" : ["Sergio Busquets", "Real Madrid C.F.", "EFL Cup", "Copa del Rey"]
}

I wish simply to return a projection which is the id and the size of the array, using Morphia in Java. In Mongo this can be done using:

{ $project: { count : {$size : "$entities"} } }

In Morphia I have attempted:

.project(projection("count",
    Projection.expression("$size", "entities")));

which returns java.lang.ClassCastException: java.lang.String cannot be cast to com.mongodb.DBObject

What is the correct equivalent expression in Morphia?

解决方案

You should use projection instead of expression like this:

.project(projection("count",projection("$size", "entities" )))

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09-13 01:07