Many scenarios
come in our programming where we need to calculate the number of the item of an array. To understand the need of the query I am creating a scenario,
suppose we have a collection named BLOG, and this collection contains the
following fields
Now, suppose BLOG collection contains the following data
"Title"
"Description"
"CreatedDate"
"Label"
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Now, suppose BLOG collection contains the following data
{
"_id" : 1, "Title" : "Title 1",
"Description" : "Desc 1", "CreatedDate" :
"2014-01-01T23:28:56.782Z", "Label" :
["MongoDB","Index","Array","Get Array
Items"]}
{
"_id" : 2, "Title" : "Title 2",
"Description" : "Desc 2", "CreatedDate" :
"2014-01-02T20:28:56.782Z", "Label" : ["SQL
Server","Merge Query"]}
{
"_id" : 3, "Title" : "Title 3",
"Description" : "Desc 3", "CreatedDate" :
"2014-01-03T08:28:56.782Z", "Label" :
["AngularJS"]}
{
"_id" : 4, "Title" : "Title 4",
"Description" : "Desc 4", "CreatedDate" :
"2014-01-04T23:28:56.782Z", "Label" :
["NodeJS","Mongoose"]}
{
"_id" : 5, "Title" : "Title 5", "Description"
: "Desc 5", "CreatedDate" :
"2014-01-05T23:28:56.782Z", "Label" : []}
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Scenario wise query
1- Suppose I want to select those
records which Label is contains the 2 attribute
db.blog.find({"Label":
{ $size : 2 }});
Result:
{"_id":2.0,"Title":"Title
2","Description":"Desc
2","CreatedDate":"2014-01-02T20:28:56.782Z","Label":["SQL
Server","Merge Query"]}
{"_id":4.0,"Title":"Title
4","Description":"Desc
4","CreatedDate":"2014-01-04T23:28:56.782Z","Label":["NodeJS","Mongoose"]}
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2- Apply projection means I want to
select some particular fields from the document
db.blog.find({"Label":
{ $size : 2 }},{Title:1,_id:1});//here I want to project Title and _id
Result:
{"_id":2.0,"Title":"Title
2"}
{"_id":4.0,"Title":"Title
4"}
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3- Document who have more than 2
records, for these type of scenario we have to apply an aggregation function
db.blog.aggregate([
{
$project:
{
Title: 1,
_id: 1,
numberOfLabel:
{
$cond:
{
if :
{
$isArray:
"$Label"
},
then:
{
$size: "$Label"
},
else
: "0"
}
}
}
},
{
$match:
{
"numberOfLabel":
{
$gt: 2
}
}
}
]);
Result:
{"_id":1.0,"Title":"Title
1","numberOfLabel":NumberInt(4)}
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4- Another query with the find function
db.blog.find(
{Label : {$exists:true}, $where:'this.Label.length>1'},{Title:1,_id:1} );
Result:
{"_id":1.0,"Title":"Title
1"}
{"_id":2.0,"Title":"Title
2"}
{"_id":4.0,"Title":"Title
4"}
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