Passing Variables to a MongoDB View - mongodb

Question

i'm trying to make views in MongoDB to avoid unnecessary returns. In the documentation says that the aggregation functions can take variables with a double dollar sign, taking this in mind i have created a view that in this example should take one variable to filter customerIds and group the results to sum the payments of different documents.
Example:
db.createView(
"viewName",
"myCollection",
[{
$match: { "customerId": "$$customerId", }
},{
$group: {
_id: null,
total: "$amount",
}
}]
)
The view is created OK and if i put some valid customerId in the aggregation function that works ok, but i don't have the slightest idea how to execute the view and pass the customerID that i need.
Any ideas? The mongodb documentation does not help me in this situation and i really need to create this as a view, since there are many applications that will connect to this view(s).
I have tried:
db.viewName.find({customerId: "some valid id"});

Answer 1

You can access it just like a collection, for example I am creating a view via:
db.runCommand({
create: 'AuthorsView',
viewOn: 'authors',
pipeline: [{
"$group": {
"_id": "$email",
"count": {
"$sum": 1
}
}
}]
})
Since this is now an existing view I can simply do:
db.getCollection('AuthorsView').find({})
To see all the documents or to add more parameters to the find
Not sure what you mean by passing variables since views are just like collections ... you run queries against them via find & aggregate.

Answer 2

First, you can't pass variables to $match without $expr. There is no error because "$$..." is interpreted as a string.
Second, If we fix things like this:
db.createView(
"viewName",
"myCollection",
[{
$match: {$expr:{$eq:["$customerId","$$customerId"]}}, }
},{
$group: {
_id: null,
total: "$amount",
}
}]
)
But $$ is not a system variable, so this won't work. You could pass a system variable like $$ROOT or a field path $field.path; the user-defined variables are made up of system variables or collection data.