Mongoose require model in model - express

Question

i want to use a static-method from another model in a model. But when i require the other model and call the function:
TypeError: Cannot read property 'isCalculated' of undefined
so.. is it not possible to require a model in another model? :(
The models are in the same folder, so I call
var Confirmation = require("./confirmation").Confirmation;
and export in confirmation:
module.exports = {
Confirmation: mongoose.model('Confirmation', confirmationSchema)
};
Thanks :)
Update: The code:invoice.js
var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var autoIncrement = require('mongoose-auto-increment');
var Moment = 'Moment';
var moment = require('moment');
var Currency = mongoose.Types.Currency;
var Confirmation = require("./confirmation.js");
autoIncrement.initialize(mongoose.connection);
var invoiceSchema = new Schema({...}); //invoiceSchema
invoiceSchema.statics.insert= function(invoice, cb) {
....
Confirmation.isCalculated(..) //error here
....
}
module.exports = mongoose.model("Invoice", invoiceSchema);
And the code of: confirmation.js
var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var Invoice = require("./invoice.js");
var confirmationSchema = new Schema({...});
confirmationSchema.statics.isCalculated = function(pid, aid, cb) {...};
module.exports = mongoose.model('Confirmation', confirmationSchema)

Answer 1

My guess would be that because of your circular imports (confirmation.js requires invoice.js which requires confirmation.js...), the Confirmation model in your invoice.js isn't materialized properly.
I think you should remove the circular imports altogether and dynamically get a reference to "the other" model using mongoose.model(), like so:
invoiceSchema.statics.insert = function(invoice, cb) {
....
mongoose.model('Confirmation').isCalculated(..)
....
}