MongoDB查询以计算文档中数组项的数量并显示在新字段中
要计算文档中数组项的数量，请在MongoDB中使用$size。让我们创建一个包含文档的集合-
> db.demo703.insertOne({"ListOfSubject":["MySQL","MongoDB"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea6ebaf551299a9f98c93b4")
}
> db.demo703.insertOne({"ListOfSubject":["Java"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea6ebb5551299a9f98c93b5")
}
> db.demo703.insertOne({"ListOfSubject":["C","C++","Python"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea6ebbf551299a9f98c93b6")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo703.find();
这将产生以下输出-
{ "_id" : ObjectId("5ea6ebaf551299a9f98c93b4"), "ListOfSubject" : [ "MySQL", "MongoDB" ] }
{ "_id" : ObjectId("5ea6ebb5551299a9f98c93b5"), "ListOfSubject" : [ "Java" ] }
{ "_id" : ObjectId("5ea6ebbf551299a9f98c93b6"), "ListOfSubject" : [ "C", "C++", "Python" ] }以下是查询以计算数组项的数量-
>db.demo703.aggregate({$project:{NumberOfItemsInEachArray:{$size:"$ListOfSubject"}}}).pretty();这将产生以下输出-
{
"_id" : ObjectId("5ea6ebaf551299a9f98c93b4"),
"NumberOfItemsInEachArray" : 2
}
{
"_id" : ObjectId("5ea6ebb5551299a9f98c93b5"),
"NumberOfItemsInEachArray" : 1
}
{
"_id" : ObjectId("5ea6ebbf551299a9f98c93b6"),
"NumberOfItemsInEachArray" : 3
}