MongoDB: elemMatch match the last element in an array - mongodb

Question

I have the data like below:
{
"order_id" : 1234567,
"order_pay_time" : 1437373297,
"pay_info" : [
{
"pay_type" : 0,
"pay_time" : 1437369046
},
{
"pay_type" : 0,
"pay_time" : 1437369123
},
{
"pay_type" : 0,
"pay_time" : 1437369348
}
]}
what I want to get is the last payment is of type 1, but $elemMatch just match the list where pay_type:1 exists, how can I match the orders which last payment is of "pay_type" : 1

Answer 1

You can use aggregation to get expected output. The query will be like following:
db.collection.aggregate({
$unwind: "$pay_info"
}, {
$match: {
"pay_info.pay_type": 1
}
}, {
$group: {
_id: "$_id",
"pay_info": {
$push: "$pay_info"
},
"order_id": {
$first: "$order_id"
},
"order_pay_time": {
$first: "$order_pay_time"
}
}
})
Moreover if you want latest pay_info.pay_time then you can sort it by descending order with limit 1, some what like following:
db.collection.aggregate({
$unwind: "$pay_info"
}, {
$match: {
"pay_info.pay_type": 1
}
}, {
$sort: {
"pay_info.pay_time": -1
}
}, {
$limit: 1
}, {
$group: {
_id: "$_id",
"pay_info": {
$push: "$pay_info"
},
"order_id": {
$first: "$order_id"
},
"order_pay_time": {
$first: "$order_pay_time"
}
}
})
Edit
Also you can use $redact to avoid $unwind like following:
db.collection.aggregate({
$match: {
"pay_info": {
$elemMatch: {
"pay_type": 1
}
}
}
}, {
$sort: {
"pay_info.pay_time": -1
}
}, {
$limit: 1
}, {
$redact: {
$cond: {
if: {
$eq: [{
"$ifNull": ["$pay_type", 1]
}, 1]
},
then: "$$DESCEND",
else: "$$PRUNE"
}
}
}).pretty()

Answer 2

Just found this thread for a similar problem I've had.
I ended up doing this, maybe that will be of interest to someone:
db.collection.find({
$where: function(){
return this.pay_info[this.pay_info.length-1].pay_type === 1
}
})