How to convert ObjectID to string in $ lookup (aggregate)
I have two collections, article and comments, articleId
in the comments is the foreign key _id
in the article.
db.collection('article').aggregate([
{
$lookup: {
from: "comments",
localField: "_id",
foreignField: "articleId",
as: "comments"
}
},
...
])
but it does not work, because _id
in the article - it's ObjectID
a articleId
- it's a string.
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You can achieve this using aggregation which simply converts the string id to mongoose objectId $toObjectId
db.collection('article').aggregate([
{ "$lookup": {
"from": "comments",
"let": { "article_Id": "$_id" },
"pipeline": [
{ "addFields": { "articleId": { "$toObjectId": "$articleId" }}},
{ "$match": { "$expr": { "$eq": [ "$articleId", "$$article_Id" ] } } }
],
"as": "comments"
}}
])
Or using aggregation $toString
db.collection('article').aggregate([
{ "addFields": { "article_id": { "$toString": "$_id" }}},
{ "$lookup": {
"from": "comments",
"localField": "article_id",
"foreignField": "articleId",
"as": "comments"
}}
])
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You can directly use $toString
on the field _id
:
db.collection('article').aggregate([
{
$lookup: {
from: "comments",
localField: { $toString : "_id" },
foreignField: "articleId",
as: "comments"
}
},
...
])
But MongoDB 3.6
does not support type conversion inside the aggregation pipeline. So $toString
and $convert
will only work with MongoDB 4.0
.
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