mongodb. how map strings to integers from another collection - mongodb

Question

I have collection with documents, for example:
[
{
'id':'1'
'some_field':'test',
'rates':[
{'user_id':'12','rate':'very_good'},
{'user_id':'13','rate':'very_good'}
{'user_id':'14','rate':'bad'},
{'user_id':'15','rate':'normal'}
]
}
]
And i have collection with values of rates in string:
[
{
"rate_name" : "bad",
"rate_value" : 1
},
{
"rate_name" : "normal",
"rate_value" : 2
},
{
"rate_name" : "very_good",
"rate_value" : 3
},
]
I need map data from first collection from array rates with value from second collection and group this values to new field.
For example:
[
{
'id':'1'
'some_field':'test',
'rates':[3,3,1,2]
]
}
]
How i can do this?

Answer 1

Here's one way you could do it if "$getField" is available (MongoDB server version 5.0 and above). See below for another option.
db.ratings.aggregate([
{
"$lookup": {
"from": "ratingScale",
"as": "ratingScale",
"pipeline": []
}
},
{
"$set": {
"rates": {
"$map": {
"input": "$rates",
"as": "rate",
"in": {
"$getField": {
"field": "rate_value",
"input": {
"$first": {
"$filter": {
"input": "$ratingScale",
"as": "nameValue",
"cond": {"$eq": ["$$rate.rate", "$$nameValue.rate_name"]}
}
}
}
}
}
}
}
}
},
{"$unset": "ratingScale"}
])
Try it on mongoplayground.net.
If "$getField" is unavailable, here's another way you could do it.
db.ratings.aggregate([
{
"$lookup": {
"from": "ratingScale",
"as": "ratingScale",
"pipeline": []
}
},
{
"$set": {
"rates": {
"$map": {
"input": "$rates",
"as": "rate",
"in": {
"$first": {
"$filter": {
"input": "$ratingScale",
"as": "nameValue",
"cond": {"$eq": ["$$rate.rate", "$$nameValue.rate_name"]}
}
}
}
}
}
}
},
{
"$set": {
"rates": {
"$map": {
"input": "$rates",
"as": "rate",
"in": "$$rate.rate_value"
}
}
}
},
{"$unset": "ratingScale"}
])
Try it on mongoplayground.net.

Answer 2

You can do a plain $unwind then $lookup approach.
db.col1.aggregate([
{
$unwind: "$rates"
},
{
"$lookup": {
"from": "col2",
"localField": "rates.rate",
"foreignField": "rate_name",
"as": "rates"
}
},
{
"$unwind": "$rates"
},
{
$group: {
_id: "$_id",
some_field: {
$first: "$some_field"
},
rates: {
$push: "$rates.rate_value"
}
}
}
])
Mongo Playground