How to query to get specific objects from array of objects? - mongodb

Question

Currently I am learning mongodb. Suppose I have one collection named post in mongodb which data is :
[{
id: 123,
uId: 111,
msg: 'test 1',
attachments: [
{name: 'attach1', url: 'https://example.com', isDeleted: false},
{name: 'attach2', url: 'https://example.com', isDeleted: true}
]
},
{
id: 456,
uId: 222,
msg: 'test 2',
attachments: [
{name: 'attach1', url: 'https://example.com', isDeleted: true}
]
},
{
id: 789,
uId: 333,
msg: 'test 3',
attachments: [
{name: 'attach1', url: 'https://example.com', isDeleted: false}
]
}]
Now i want the result of all post where attachments.isDeleted is false which look like :
[{
id: 123,
uId: 111,
msg: 'test 1',
attachments: [
{name: 'attach1', url: 'https://example.com', isDeleted: false}
]
},
{
id: 456,
uId: 222,
msg: 'test 2',
attachments: []
},
{
id: 789,
uId: 333,
msg: 'test 3',
attachments: [
{name: 'attach1', url: 'https://example.com', isDeleted: false}
]
}]
I tried this db.post.find({attachments: {$elemMatch: {isDeleted: false}}}) but it is not working. I have taken help from [https://stackoverflow.com/questions/62953855/how-get-query-for-array-of-objects-in-mongodb]

Answer 1

I think this is what you are looking for. It uses the Mongodb aggregation framework. You can take a look the documentation to see details. In brief, the $project stage allow us select fields to show or to hide, and it admits calculated fields. We calculated a new attachments field using $filter stage with a the given condition (isDeleted equals to false).
db.collection.aggregate([
{
"$project": {
_id: 1,
uId: 1,
msg: 1,
attachments: {
"$filter": {
"input": "$attachments",
"as": "attachment",
"cond": {
"$eq": [
"$$attachment.isDeleted",
false
]
}
}
}
}
}
])
You can try it in: https://mongoplayground.net/p/YDvpkbZIZ2H
Note that I changed id by _id, but it was only for style purpose.
Hope I helped.