Find specific field in MongoDB document based on condition - mongodb

Question

I have the following MongoDB documents like this one:
{
"_id": "ABC",
"properties":
[
{
"_id": "123",
"weight":
{
"$numberInt": "0"
},
"name": "Alice"
},
{
"_id": "456",
"weight":
{
"$numberInt": "1"
},
"name": "Bob"
},
{
"_id": "789",
"weight":
{
"$numberInt": "1"
},
"name": "Charlie"
}
]
}
And I would like to find the _id of the property with name "Alice", or the _id of the property with "$numberInt": "0".
I'm using pymongo.
The following approach:
from pymongo import MongoClient
mongo_client = MongoClient("mymongourl")
mongo_collection = mongo_client.mongo_database.mongo_collection
mongo_collection.find({'properties.name': 'Alice'}, {'properties': 1})[0]['_id']
Gives the very first _id ("123")
But since I filtered for the document, if Alice was in the second element of the properties array (_id: "456") I would have missed her.
Which is the best method to find for the specific _id associated with the element with the specified name?

Answer 1

You can simply use $reduce to iterate through the properties array. Conditionally store the _id field if it matches your conditions.
db.collection.aggregate([
{
"$addFields": {
"answer": {
"$reduce": {
"input": "$properties",
"initialValue": null,
"in": {
"$cond": {
"if": {
$or: [
{
$eq: [
"$$this.name",
"Alice"
]
},
{
$eq: [
"$$this.weight",
0
]
}
]
},
"then": "$$this._id",
"else": "$$value"
}
}
}
}
}
}
])
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