Database and model design - mongoose

A new user Alan Tang, on Stackoverflow posted a question regarding the top-level design of schemas for database.
The link to the question is here:
https://stackoverflow.com/questions/70345194/node-js-model-with-inner-nested-array/70345760#70345760
More detail information is below:

I have a question about my collection design.
Current design

Code (Part-I):

Every time, if a customer purchases a new item, it will push the history into the "purchase_history".
The purpose of "purchase_history" is to let them check their own history.
Is this a good idea? or If you have a good idea, please few free to share.
Thank you

I answered to the above question like this:
I have created 2 different schemas, one for the customer and another for the purchase. Please follow the standard procedure to keep all the service files, model files, controller files in separate folders.
The below is the customer model.

customer.model.js
Code (Part-II):

We have the purchase model as:

purchase.model.js
Code (Part-III):

Here, we can see the customer data is stored in customer collection and the purchase data stored in the purchase collection. Each purchase record has a reference field 'customerId' which is the customer's unique identitifer. This field is defined in the purchase model. The customer's purchase history can be fetched by quering for the customerId field.
We can create an api for fetching customer's purchases as:

purchase.service.js
Code (Part-IV):

DESIGN PRINCIPLE:
The design principle followed here is to avoid duplication as suggested by a Senior developer. It is not good practice to store the same value in different collections, the purchase data being stored in the customer collection as in Alan’s schema.