Is there a way to use a value recently gotten to look for other? in MongoDB - mongodb

Question

I want to make a query and use a value gotten to look for other in the same query.
My collection is like this:
{
"houses": {
123: {
"color": "white",
"location": "California"
},
124: {
"color": "blue",
"location": "Las Vegas"
}
},
"owners": {
"Anne": {
"house": 124,
},
"Jake": {
"house": 123
}
}
}
Before doing the query I will know just the name of the owner and I would like to get the house information (color, location).
What I'm asking is that if there's a way of using the house number to get the house info in the same query. Something like this:
db.collection.aggregate([
{'$project' {'houses': 1, 'house_number': '$owners.Anne.house'}},
{'$project': {'house_info': 'houses.$house_number':1}}
])
I tried making the house number a string and concat it with $houses but mongo doesn't let me concat the $ symbol.
I am avoiding to make two queries, one for getting the house number and then a second one for getting the house information.
Could someone please help me with this? Sorry if I can't explain myself very well, english isn't my native language.

Answer 1

Quite possible with the use of $objectToArray and $filter operators. The $objectToArray is to convert the houses object / document to an array of key/value
properties. With this array you can filter using the '$owners.Anne.house' value.
Take for instance this aggregate pipeline:
db.collection.aggregate([
{ '$project': {
'house_info': {
'$filter': {
'input': { '$objectToArray': '$houses' },
'cond': {
'$eq': ['$owners.Anne.house', '$$this.k']
}
}
}
} }
])
The result is something like :
{
'house_info': [
{
k: '124',
v: {
"color": "blue",
"location": "Las Vegas"
}
}
]
}
To get just the data document
{
"color": "blue",
"location": "Las Vegas"
}
Add a further projection pipeline stage that uses $arrayElemAt operator as
db.collection.aggregate([
{ '$project': {
'house_array': {
'$filter': {
'input': { '$objectToArray': '$houses' },
'cond': {
'$eq': ['$owners.Anne.house', '$$this.k']
}
}
}
} },
{ '$project': {
'house_info': {
'$arrayElemAt': ['$house_array', 0]
}
} }
])