3. Schemas and Models

Schemas vs. Models

• A schema in Mongoose defines the structure of a document within a collection. Think of it like defining the columns in a table in PostgreSQL but in your Node.js application instead of directly in the database.
• A model is the JavaScript object you use to interact with the database based on a schema.

Key Note: Schemas Aren’t in the Database

Unlike PostgreSQL, where table definitions (schemas) are enforced by the database itself, Mongoose schemas exist only in your application code. MongoDB itself is schema-less, so it doesn’t enforce these rules. Mongoose schemas act as a layer of validation before data is stored.

• In PostgreSQL: You create tables with strict constraints:

  CREATE TABLE students (
    id SERIAL PRIMARY KEY,
    first_name TEXT NOT NULL,
    last_name TEXT NOT NULL,
    email TEXT UNIQUE NOT NULL,
    enrollment_date DATE
  );

• In Mongoose: You define schemas in code:

  const studentSchema = new mongoose.Schema({
    firstName: { type: String, required: true },
    lastName: { type: String, required: true },
    email: { type: String, required: true, unique: true },
    enrollmentDate: { type: Date, required: true },
  });
  
  const Student = mongoose.model('Student', studentSchema);

Why Use Schemas in Mongoose?

Schemas let you:

• Define required fields, data types, and validation rules.
• Add custom methods for processing data.
• Establish relationships between documents.