MongoDB Update Statement Set Value From Other Table

I have a background in SQL server and I'm migrating some data over to MongoDB for a proof of concept project using Mongoose as the driver. I want to add a DB ref to another collection. A basic example of what I'm trying to do here:

Project Schema:

{
    projectNo: String,
    projectTitle: String
}

Document Schema:

{
    projectId: { type: mongoose.Schema.ObjectId, ref: 'project',
    projectNo: String,
    documentTitle: String
}

I want to add a new key to the Document schema that references the _id of the project collection. If I was doing this in SQL Server I'd do this:

UPDATE document
SET projectId = project._id 
FROM project 
WHERE document.projectNo = project.projectNo

Can anyone point me in the right direction? Thank you!

Kevin