Mix data from MySQL in MongoDB query? - mysql

Question

I have the following query in MongoDB, which selects the last 10 conversations a user has participated in. They are sorted based on the last message in a conversation. I have a way to paginate between the results using $gt in the match statement. For now, this is commented out.
I limited the number of members to 3 (using slice), because a group conversation can contain, for example, 30, 40, 50, ... members.
The problem: I have a MySQL database to store relational information. So I have a table users and a table followers, to show who is following each other. I want to include this information in a way into MongoDB, so I can find 3 members I am following, to show relevant profile pictures of known people. Let's say I have a conversation with 50 members, then I want to retrieve 3 people I follow (if any), so I see familiar profile pictures. Is this possible in a way, or is my wish not possible?
query
db.getCollection('conversations').aggregate([
{
$lookup: {
foreignField: "c_ID",
from: "messages",
localField: "_id",
as: "messages"
}
},
{
"$unwind": "$messages"
},
{
"$sort": {
"messages.t": -1
}
},
{
"$group": {
"_id": "$_id",
"lastMessage": {
"$first": "$messages"
},
"allFields": {
"$first": "$$ROOT"
}
}
},
{
"$replaceRoot": {
"newRoot": {
"$mergeObjects": [
"$allFields",
{
"lastMessage": "$lastMessage"
}
]
}
}
},
{
$project: {
messages: 0
}
},
{
$match: {
"members.uID": "1",
//"lastMessage.t": { $gt: ISODate("2020-02-04 20:38:02.154Z") }
}
},
{
$sort: { "lastMessage.t": 1 }
},
{
$limit: 10
},
{
$project: {
members: {
$slice: [ {
$filter: {
input : "$members", as : "member", cond : {
$ne : ["$$member.uID" , "1"]
}
}
}, 3 ]
}
}
},
])
conversations document
{
"_id" : ObjectId("5e35f2c840713a43aeeeb3d9"),
"members" : [
{
"uID" : "1",
"j" : 1580580922
},
{
"uID" : "4",
"j" : 1580580922
},
{
"uID" : "5",
"j" : 1580580922
}
]
}
messages document
{
"_id" : ObjectId("5e35ee5f40713a43aeeeb1c5"),
"c_ID" : ObjectId("5e35f2c840713a43aeeeb3d9"),
"fromID" : "1",
"msg" : "What's up?",
"t" : 1580591922,
"d" : {
"4" : 1580592039
},
"r" : {
"4" : 1580592339
}
}