Merge arrays by matching similar values in mongodb - mongodb

Question

This is an extension of the below question.
Filter arrays in mongodb
I have a collection where each document contains 2 arrays as below.
{
users:[
{
id:1,
name:"A"
},
{
id:2,
name:"B"
},
{
id:3,
name:"C"
}
]
priv_users:[
{
name:"X12/A",
priv:"foobar"
},
{
name:"Y34.B",
priv:"foo"
}
]
}
From the linked question, I learnt to use $map to merge 2 document arrays. But I can't figure out to match users.name to priv_users.name to get below output.
{
users:[
{
id:1,
name:"A",
priv:"foobar"
},
{
id:2,
name:"B",
priv:"foo"
},
{
id:3,
name:"C"
}
]
}
users.name and priv_users.name don't have a consistent pattern, but users.name exists within priv_users.name.
MongoDB version is 4.0

Answer 1

This may not be as generic but will push you in the right direction. Consider using the operators $mergeObjects to merge the filtered document from the priv_users array with the document in users.
Filtering takes the $substr of the priv_users name field and compares it with the users name field. The resulting pipeline will be as follows
db.collection.aggregate([
{ '$addFields': {
'users': {
'$map': {
'input': '$users',
'in': {
'$mergeObjects': [
{
'$arrayElemAt': [
{
'$filter': {
'input': '$priv_users',
'as': 'usr',
'cond': {
'$eq': [
'$$this.name',
{ '$substr': [
'$$usr.name', 4, -1
] }
]
}
}
},
0
]
},
'$$this'
]
}
}
}
} }
])
If using MongoDB 4.2 and newer versions, consider using $regexMatch operator for matching the priv_users name field with the users name field as the regex pattern. Your $cond operator now becomes:
'cond': {
'$regexMatch': {
'input': '$$usr.name',
'regex': '$$this.name',
'options': "i"
}
}