The problem I am facing is below:
I have a MongoDB document whose structure is as follows
"name": "XYZ",
"array":[
{
"value": "Alpha"
},
{
"value": "Beta"
},
{
"value": "Alpha"
},
]
and I have to count how many objects have value Alpha. I have tried the following two queries but both only give me value 1.
db.current_database.find({array: {$elemMatch: {value: "Alpha"}}}).count()
db.current_database.find({'array.value': 'Alpha'}).count()
The find collection method returns documents, not fragments.
A few options to count occurrances of elements in an array:
Most languages provide a method to filter/reduce/count elements in an array, so this should be fairly straightforward on the client side.
The MongoDB aggregation framework provides $reduce, $filter, $size, $group, $unwind, and a few other operators that might be useful in this situation.
One possible solution using $reduce:
db.current_database.aggregate([
{$match: {"array.value": "Alpha"}},
{$addFields:{
count: {
$reduce: {
input: "$array",
initialValue: 0,
in: {
$cond: {
if: {$eq: ["$$this.value", "Alpha"]},
then: {$sum: ["$$value", 1]},
else: "$$value"
}
}
}
}
}}
])
$project to limit which fields are returned. The driver or shell may have convenience functions to show the value of one field. For instance, adding .next().count to that sample query in the shell will return just the value of the count field from the first document.