即使某些值为null，也要实现MongoDB $ concatArrays？
为此，请使用聚合框架以及$ifNull运算符。聚合中的$concatArrays用于连接数组。首先让我们创建一个包含文档的集合-
>db.concatenateArraysDemo.insertOne({"FirstSemesterSubjects": ["MongoDB","MySQL","Java"],"SecondSemesterSubjects":["C","C++",]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd687707924bb85b3f4895c")
}
> db.concatenateArraysDemo.insertOne({"FirstSemesterSubjects":["C#","Ruby","Python"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd687927924bb85b3f4895d")
}
>db.concatenateArraysDemo.insertOne({"FirstSemesterSubjects":["HTML","CSS","Javascript"],"SecondSemesterSubjects":["CSS","Javascript"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd687bb7924bb85b3f4895e")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.concatenateArraysDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cd687707924bb85b3f4895c"),
"FirstSemesterSubjects" : [
"MongoDB",
"MySQL",
"Java"
],
"SecondSemesterSubjects" : [
"C",
"C++"
]
}
{
"_id" : ObjectId("5cd687927924bb85b3f4895d"),
"FirstSemesterSubjects" : [
"C#",
"Ruby",
"Python"
]
}
{
"_id" : ObjectId("5cd687bb7924bb85b3f4895e"),
"FirstSemesterSubjects" : [
"HTML",
"CSS",
"Javascript"
],
"SecondSemesterSubjects" : [
"CSS",
"Javascript"
]
}以下是即使为null时也要连接数组的查询-
> db.concatenateArraysDemo.aggregate([
{$project: {
ConcatenateArrays: { '$concatArrays': [
{$ifNull: ['$FirstSemesterSubjects', []]},
{$ifNull: ['$SecondSemesterSubjects', []]}
] }
} }
]);这将产生以下输出-
{ "_id" : ObjectId("5cd687707924bb85b3f4895c"), "ConcatenateArrays" : [ "MongoDB", "MySQL", "Java", "C", "C++" ] }
{ "_id" : ObjectId("5cd687927924bb85b3f4895d"), "ConcatenateArrays" : [ "C#", "Ruby", "Python" ] }
{ "_id" : ObjectId("5cd687bb7924bb85b3f4895e"), "ConcatenateArrays" : [ "HTML", "CSS", "Javascript", "CSS", "Javascript" ] }