How to push field into embedded document

Model

var ModuleSchema = new Schema({
    name: String,
    types: [{
        name: String,
        functions: [{
            name: String
        }]
    }]    
});

var CoverageSchema = new Schema({
    name: String,
    modules: [{
        name: String,
        types: [{
            name: String,
            functions: [{
                name: String,
                coverages: [{
                    blocksCovered: Number
                }]
            }]
        }]    
});

mongoose.model('Module', ModuleSchema);
mongoose.model('Coverage', CoverageSchema);

coverage.server.controller.js

coverage.modules = []; // Mongoose CoverageSchema existing object.

Module.findOne(...).exec(function(err, module) {
     // If not error
     coverage.modules.push({
         name: module.name, 
         types: module.types
     });
});

// How do I add 'coverage.modules.types.functions.coverages: [] ?
coverage.modules.forEach(function(mod){
    mod.types.forEach(function(type){
          type.functions.forEach(function(func){
                func.coverages.push({blocksCovered: 0}); // Error: coverages undefined.
          });
    });
});

coverage.save(function (err) ...)

I am able to save coverage object with inserted modules from ModuleSchema. However cannot add after it modules.types.functions.coverages embedded document before save.