Core Mental Model & Data Modeling

A rough mapping:

PostgreSQL	MongoDB	Important difference
Table	Collection	Collection does not require every record to have identical fields
Row	Document	Document can contain nested objects and arrays
Column	Field	Fields may differ between documents
Primary key	`_id`	Every normal document has one; uniquely indexed by default
JSONB	BSON document	MongoDB's entire primary data model is document-oriented

In PostgreSQL, you normally start by thinking:

What are my entities and relationships? How should I normalize them?

In MongoDB, you more often start with:

What data does the application usually read and modify together?

Imagine an asset-heavy product:

BSON

MongoDB stores documents as BSON, not plain JSON. BSON supports nested documents/arrays and database-oriented types such as ObjectId, Date, int64, Decimal128, and binary data.

type Product struct {
	ID     primitive.ObjectID `bson:"_id,omitempty"`
	SKU    string             `bson:"sku"`
	Name   string             `bson:"name"`
	Assets []Asset            `bson:"assets"`
}

{
    "_id": ObjectId("..."),
    "sku": "TSHIRT-001",
    "name": "Black T-Shirt",

    "assets": [
        {
            "asset_id": "asset-101",
            "position": 1,
            "status": "ready",
            "variants": {
                "thumbnail": {
                    "width": 200,
                    "height": 200,
                    "s3_key": "products/101/thumb.webp"
                },
                "large": {
                    "width": 1600,
                    "height": 1600,
                    "s3_key": "products/101/large.webp"
                }
            }
        }
    ]
}

`_id`

Every document in a normal MongoDB collection has an _id that uniquely identifies it. If you don't provide one, drivers normally generate an ObjectId.

Flexible schema ≠ no schema

MongoDB has a flexible document schema rather than an absence of schema.

Without discipline, flexibility can actually create bugs:

{ "status": "ready" }
{ "status": 1 }
{ "processing_status": "READY" }

Document ≈ Aggregate

If these things are normally: