Defining schemas and models in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Mongoose schema validation?

Consider this Mongoose schema for a User model. What happens if you try to save a user with an empty email field?

Express

const mongoose = require('mongoose');
const userSchema = new mongoose.Schema({
  email: { type: String, required: true },
  name: String
});
const User = mongoose.model('User', userSchema);

const user = new User({ name: 'Alice' });
user.save().catch(err => console.log(err.message));

A It throws a validation error because email is required

B It saves but email is saved as an empty string

C It throws a syntax error due to missing email

D It saves successfully with email as undefined

💡 Hint

Check the 'required' property in the schema definition.

The 'required: true' in the schema means Mongoose will throw a validation error if the email field is missing when saving.

📝 Syntax

intermediate

Which option correctly defines a Mongoose schema with a nested object?

You want to define a schema for a BlogPost with a nested author object containing name and age. Which code snippet is correct?

A const blogSchema = new mongoose.Schema({ title: String, author: { type: Object, name: String, age: Number } });

B const blogSchema = new mongoose.Schema({ title: String, author: mongoose.Schema({ name: String, age: Number }) });

C const blogSchema = new mongoose.Schema({ title: String, author: new mongoose.Schema({ name: String, age: Number }) });

D const blogSchema = new mongoose.Schema({ title: String, author: { name: String, age: Number } });

💡 Hint

Nested objects can be defined directly as plain objects inside the schema.

Option D correctly defines a nested object by using a plain object for 'author'. Options A and C misuse Schema constructor, and D has invalid syntax.

🔧 Debug

advanced

Why does this Mongoose model throw a 'Missing schema' error?

You have this code snippet. Why does it throw an error when trying to create a model?

Express

const mongoose = require('mongoose');
const userSchema = mongoose.Schema({ username: String });
const User = mongoose.model('User');

A Because the schema is missing a required field

B Because the model is created without passing the schema as the second argument

C Because mongoose.Schema is not a function

D Because the model name 'User' is reserved

💡 Hint

Check the parameters passed to mongoose.model.

The model function requires the schema as the second argument. Omitting it causes a 'Missing schema' error.

❓ state_output

advanced

What is the value of 'user.isNew' after saving a new document?

Given this code, what will be the value of 'user.isNew' after calling save() successfully?

Express

const mongoose = require('mongoose');
const userSchema = new mongoose.Schema({ name: String });
const User = mongoose.model('User', userSchema);

async function run() {
  const user = new User({ name: 'Bob' });
  console.log(user.isNew);
  await user.save();
  console.log(user.isNew);
}
run();

A true before save, false after save

B false before save, true after save

C true both before and after save

D false both before and after save

💡 Hint

Check the meaning of 'isNew' property in Mongoose documents.

'isNew' is true for unsaved documents and becomes false after saving to the database.

🧠 Conceptual

expert

Which option best describes the purpose of Mongoose middleware in schemas?

What is the main use of middleware functions in Mongoose schemas?

A To define custom validation rules for schema fields

B To automatically create indexes on schema fields

C To run functions before or after certain schema methods like save or remove

D To convert schema definitions into JSON objects

💡 Hint

Think about hooks that run during document lifecycle events.

Middleware in Mongoose allows running code before or after events like saving or removing documents.