How to connect between two collection pr object on json file?

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JSON

MongoDB

mongoose

i have two diffrent object on my json file and i want to connect between them with the attribute of systemName.
how to do so?

this is how the event object look like:
and i want to get the systemName from the Systems object the contain all the systems name List.

{
    "_id" : ObjectId("5ad8764465639315a4e54e39"),
    "yellowEvents" : 1,
    "redEvents" : 1,
    "commands" : "test",
    "systemName" : "armyEG",
    "__v" : 0
}

this is how the System object look like :

{
    "_id" : ObjectId("5ad86abc6837621e309671a3"),
    "systemName" : "armyTS",
    "__v" : 0
},
{
    "_id" : ObjectId("5ad86abc6837621e309671a3"),
    "systemName" : "armyIL",
    "__v" : 0
},
{
    "_id" : ObjectId("5ad86abc6837621e309671a3"),
    "systemName" : "armyEG",
    "__v" : 0
},
{
    "_id" : ObjectId("5ad86abc6837621e309671a3"),
    "systemName" : "armyRU",
    "__v" : 0
}

What I have tried:

i have tried to do like this:

{
    "_id" : ObjectId("5ad8764465639315a4e54e39"),
    "yellowEvents" : 1,
    "redEvents" : 1,
    "commands" : "test",
    "systemName" : "System[2].systemName",
    "__v" : 0
}