Mongo DB $lookup with no common value to match - mongodb

Question

I am a novice to MongoDb and am trying to join two collections where there is no common value.
I have two collections.
collection 1 : Role
Fields : Role, UserName
collection 2 :mysite
Fields : userName ,userEmail
In collection 1:
eg :
{
'Role' :"admin"
'UserName' : "abc.efg"
}
In collection 2:
eg:
{
'userName' : "abc Mr, efg"
'userEmail' : "abc.efg#company.com"
}
The value of username is different in format so I am looking for a way to join these two collections.
Is there any way to merge these two collections please.
Kindly help on this.

Answer 1

To perform uncorrelated subqueries between two collections as well as allow other join conditions besides a single equality match, the $lookup stage has the following syntax:
{
$lookup:
{
from: <collection to join>,
let: { <var_1>: <expression>, …, <var_n>: <expression> },
pipeline: [ <pipeline to execute on the collection to join> ],
as: <output array field>
}
}
You can use the following aggregation query using the above $lookup syntax:
db.Role.aggregate([
{
"$lookup": {
"from": "mysite",
let: {
"userName": "$UserName"
},
"pipeline": [
{
$match: {
"$expr": {
"$ne": [
{
"$indexOfCP": [
"$userEmail",
"$$userName"
]
},
-1
]
}
}
}
],
"as": "mysite"
}
},
{
"$unwind": "$mysite"
},
])
MongoDB Playground
$indexOfCP searches a string for an occurrence of a substring and returns the index of the first occurrence. If the substring is not found, it returns -1.
So, in the following stage, it checks if UserName substring is present in userEmail, if not it returns -1, if present it returns the index at which the substring is located.
Hence using the expr $ne -1 , it matches all documents that have UserNamesubstring present in userEmail, ignoring the documents where given substring is not present.
{
$match: {
"$expr": {
"$ne": [
{
"$indexOfCP": [
"$userEmail",
"$$userName"
]
},
-1
]
}
}
}