Conditionally Project Matching Array Item - arrays

Question

I have a collection named questions with documents like this:
{
"formats": [
{
"language_id": 1,
"text": "question text1"
},
{
"language_id": 2,
"text": "question text 2"
}
],
"qid": "HQSRFA3T"
}
I want to write a query such that if a specific language_id is not present, then language_id with 1 should be returned by default.
I've tried two queries so far:
db.questions.aggregate([
{
$match: {
'qid': 'HQSRFA3T'
}
},
{
$project: {
formats: {
$ifNull: [
{ $filter: { input: '$formats', as: 'format', cond: {$eq: ['$$format.language_id', 3]}} },
{ $filter: { input: '$formats', as: 'format', cond: {$eq: ['$$format.language_id', 1]}} }
]
},
_id: 0
}
}
])
This query results in something like this:
{ "formats" : [ ] }
Then there's another query which is something like this:
db.questions.aggregate([ { $match: {'qid': 'HQSRFA3T'}}, { $project: {
formats: {
$filter: {
input: '$formats',
as: 'format',
cond: {
$or: [
{ $eq: ['$$format.language_id', 1] },
{ $eq: ['$$format.language_id', 3] }
]
}
}
},
_id: 0
}}])
This query returns two elements in case both language_id's are present in the array.

Answer 1

There are a "few" ways:
Ideally you have $indexOfArray from MongoDB 3.4 then you can use that in combination with $in:
db.questions.aggregate([
{ "$match": { "qid": "HQSRFA3T" } },
{ "$project": {
"formats": {
"$cond": {
"if": { "$in": [ 3, "$formats.language_id"] },
"then": {
"$arrayElemAt": [
"$formats",
{ "$indexOfArray": [ "$formats.language_id", 3 ] }
]
},
"else": {
"$arrayElemAt": [
"$formats",
{ "$indexOfArray": [ "$formats.language_id", 1 ] }
]
}
}
}}
}
])
And if all you really want is the matching "text", then a slight alteration:
db.questions.aggregate([
{ "$match": { "qid": "HQSRFA3T" } },
{ "$project": {
"text": {
"$cond": {
"if": { "$in": [ 3, "$formats.language_id"] },
"then": {
"$arrayElemAt": [
"$formats.text",
{ "$indexOfArray": [ "$formats.language_id", 3 ] }
]
},
"else": {
"$arrayElemAt": [
"$formats.text",
{ "$indexOfArray": [ "$formats.language_id", 1 ] }
]
}
}
}}
}
])
That works because if the $indexOfArray returns -1 indicating "not found" then the the $cond will branch accordingly:
Alternately, use $filter with $size:
db.questions.aggregate([
{ "$match": { "qid": "HQSRFA3T" } },
{ "$project": {
"formats": {
"$cond": {
"if": { "$gt": [
{ "$size": {
"$filter": {
"input": "$formats",
"cond": { "$eq": [ "$$this.language_id", 3 ] }
}
}},
0
]},
"then": {
"$filter": {
"input": "$formats",
"cond": { "$eq": [ "$$this.language_id", 3 ] }
}
},
"else": {
"$filter": {
"input": "$formats",
"cond": { "$eq": [ "$$this.language_id", 1 ] }
}
}
}
}
}}
])
You can even vary on the last form with $arrayElemAt to just return the "single" matching array element at position 0 if you at least have MongoDB 3.2.
db.questions.aggregate([
{ "$match": { "qid": "HQSRFA3T" } },
{ "$project": {
"formats": {
"$cond": {
"if": { "$gt": [
{ "$size": {
"$filter": {
"input": "$formats",
"cond": { "$eq": [ "$$this.language_id", 3 ] }
}
}},
0
]},
"then": {
"$arrayElemAt": [
{ "$filter": {
"input": "$formats",
"cond": { "$eq": [ "$$this.language_id", 3 ] }
}},
0
]
},
"else": {
"$arrayElemAt": [
{ "$filter": {
"input": "$formats",
"cond": { "$eq": [ "$$this.language_id", 1 ] }
}},
0
]
}
}
}
}}
])
Other alternatives on the $cond for the if condition are using $in, to match the comparison on the array elements:
"if": { "$in": [ 3, "$formats.language_id" ] }
But since that requires MongoDB 3.4, then you may as well use the $indexOfArray operator instead.
There is very little point in trying to "force" multiple matches into $filter and then ultimately looking to discard one of them, but you "can" do it with $let:
db.questions.aggregate([
{ "$match": { "qid": "HQSRFA3T" } },
{ "$project": {
"formats": {
"$let": {
"vars": {
"formats": {
"$filter": {
"input": "$formats",
"cond": {
"$or": [
{ "$eq": [ "$$this.language_id", 1 ] },
{ "$eq": [ "$$this.language_id", 3 ] }
]
}
}
}
},
"in": {
"$cond": {
"if": {
"$gt": [
{ "$size": {
"$filter": {
"input": "$$formats",
"cond": { "$eq": [ "$$this.language_id", 3 ] }
}
}},
0
]
},
"then": {
"$filter": {
"input": "$$formats",
"cond": { "$eq": [ "$$this.language_id", 3 ] }
}
},
"else": {
"$filter": {
"input": "$$formats",
"cond": { "$eq": [ "$$this.language_id", 1 ] }
}
}
}
}
}
}
}}
])
So it's there, but it's just extra work with little gain since at best the $or condition matches the "default" case and you still needed to "filter away" for only the "preferred" match anyway.