在Morphia的Mongo聚合管道中使用运算符的正确方法是什么 [英] What is the correct way to use operators in Mongo aggregation pipelines with Morphia

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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