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Implement MongoDB $concatArrays even when some of the values are null?
Use aggregate framework along with $ifNull operator for this. The $concatArrays in aggregation is used to concatenate arrays. Let us first create a collection with documents −
>db.concatenateArraysDemo.insertOne({"FirstSemesterSubjects": ["MongoDB","MySQL","Java"],"SecondSemesterSubjects":["C","C++",]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd687707924bb85b3f4895c")
}
> db.concatenateArraysDemo.insertOne({"FirstSemesterSubjects":["C#","Ruby","Python"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd687927924bb85b3f4895d")
}
>db.concatenateArraysDemo.insertOne({"FirstSemesterSubjects":["HTML","CSS","Javascript"],"SecondSemesterSubjects":["CSS","Javascript"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd687bb7924bb85b3f4895e")
}Following is the query to display all documents from a collection with the help of find() method −
> db.concatenateArraysDemo.find().pretty();
This will produce the following output −
{
"_id" : ObjectId("5cd687707924bb85b3f4895c"),
"FirstSemesterSubjects" : [
"MongoDB",
"MySQL",
"Java"
],
"SecondSemesterSubjects" : [
"C",
"C++"
]
}
{
"_id" : ObjectId("5cd687927924bb85b3f4895d"),
"FirstSemesterSubjects" : [
"C#",
"Ruby",
"Python"
]
}
{
"_id" : ObjectId("5cd687bb7924bb85b3f4895e"),
"FirstSemesterSubjects" : [
"HTML",
"CSS",
"Javascript"
],
"SecondSemesterSubjects" : [
"CSS",
"Javascript"
]
}Following is the query to concatenate arrays even when null −
> db.concatenateArraysDemo.aggregate([
{$project: {
ConcatenateArrays: { '$concatArrays': [
{$ifNull: ['$FirstSemesterSubjects', []]},
{$ifNull: ['$SecondSemesterSubjects', []]}
] }
} }
]);This will produce the following output −
{ "_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" ] }
Updated on: 30-Jul-2019
191 Views
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