Relations in Prisma in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Prisma query with relations?

Given the Prisma schema with User and Post models where User has many Posts, what will this query return?

NestJS

const userWithPosts = await prisma.user.findUnique({
  where: { id: 1 },
  include: { posts: true }
});
console.log(userWithPosts);

A An array of posts without any user data

B Only the user data without any posts included

C An object with user data and an array of all posts by that user under the 'posts' key

D A syntax error because 'include' is not valid here

💡 Hint

Think about what the 'include' option does in Prisma queries.

The 'include' option tells Prisma to fetch related records. Here, it fetches all posts related to the user with id 1 and adds them under the 'posts' key.

❓ state_output

intermediate

What is the value of 'post.author' after this Prisma query?

Given the models Post and User where Post has a relation to User as 'author', what does this code output?

NestJS

const post = await prisma.post.findUnique({
  where: { id: 5 },
  include: { author: true }
});
console.log(post.author);

A An object containing the author's user data for the post with id 5

B Undefined because 'author' is not included by default

C An array of all users in the database

D A runtime error because 'author' is not a valid relation

💡 Hint

Check how 'include' works with nested relations.

Including 'author: true' fetches the related user who wrote the post. So 'post.author' contains that user's data.

📝 Syntax

advanced

Which option correctly defines a one-to-many relation in Prisma schema?

You want to define a User who has many Posts. Which schema snippet is correct?

A

model User {
  id    Int    @id @default(autoincrement())
  posts Post[]
}

model Post {
  id       Int  @id @default(autoincrement())
  userId   Int
}

B

model User {
  id    Int    @id @default(autoincrement())
  posts Post[]
}

model Post {
  id       Int  @id @default(autoincrement())
  userId   Int
  user     User @relation(fields: [userId], references: [id])
}

C

model User {
  id    Int    @id @default(autoincrement())
  posts Post[]
}

model Post {
  id       Int  @id @default(autoincrement())
  user     User
}

D

model User {
  id    Int    @id @default(autoincrement())
  posts Post
}

model Post {
  id       Int  @id @default(autoincrement())
  userId   Int
  user     User
}

💡 Hint

Look for the correct use of @relation with fields and references.

Option B correctly defines the one-to-many relation with the foreign key 'userId' in Post and the relation decorator linking it to User's 'id'.

🔧 Debug

advanced

Why does this Prisma query throw an error?

Given the models User and Post where User has a relation to Post as 'posts', this code throws an error. What is the cause?

NestJS

const users = await prisma.user.findMany({
  where: { posts: { id: 1 } }
});

A Missing 'include' to fetch related author data

B The 'posts' field does not exist in the User model

C The 'findMany' method cannot filter by relations

D The 'where' filter uses a nested object without 'some' or 'every' for a relation array

💡 Hint

Check how to filter by relations that are arrays in Prisma.

When filtering by a relation that is an array, you must use 'some', 'every', or 'none' to specify the condition. Direct nested object causes error.

🧠 Conceptual

expert

How does Prisma handle cascading deletes in relations by default?

Consider a one-to-many relation where deleting a User should delete all their Posts. What is Prisma's default behavior if no cascade is specified?

A Prisma prevents deleting the User if related Posts exist, throwing an error

B Prisma automatically deletes all related Posts when the User is deleted

C Prisma sets the foreign key in Posts to null when the User is deleted

D Prisma ignores the relation and deletes the User without affecting Posts

💡 Hint

Think about database foreign key constraints and Prisma's default relation behavior.

By default, Prisma does not cascade deletes. It enforces foreign key constraints, so deleting a User with related Posts causes an error unless cascade is explicitly set.

Practice

1. What type of relation in Prisma connects one record to many records in another model?

easy

A. One-to-many relation

B. One-to-one relation

C. Many-to-many relation

D. No relation

Solution

1. Step 1: Understand relation types in Prisma

   Prisma supports one-to-one, one-to-many, and many-to-many relations to connect models.

2. Step 2: Identify the relation connecting one record to many

   One-to-many means one record in a model relates to multiple records in another model.

3. Final Answer:

   One-to-many relation -> Option A

4. Quick Check:

   One record linked to many = One-to-many [OK]

Hint: One record to many records means one-to-many relation [OK]

Common Mistakes:

• Confusing one-to-one with one-to-many
• Thinking many-to-many is one-to-many
• Assuming no relation means one-to-many

2. Which of the following is the correct Prisma schema syntax for a one-to-one relation between User and Profile models?

easy

A. model User {\n id Int @id @default(autoincrement())\n profiles Profile[]\n}\n\nmodel Profile {\n id Int @id @default(autoincrement())\n user User\n}

B. model User {\n id Int @id @default(autoincrement())\n profile Profile? @relation(fields: [profileId], references: [id])\n profileId Int?\n}\n\nmodel Profile {\n id Int @id @default(autoincrement())\n user User?\n}

C. model User {\n id Int @id @default(autoincrement())\n profile Profile\n}\n\nmodel Profile {\n id Int @id @default(autoincrement())\n user User\n}

D. model User {\n id Int @id @default(autoincrement())\n profileId Int\n}\n\nmodel Profile {\n id Int @id @default(autoincrement())\n}

Solution

1. Step 1: Check one-to-one relation syntax

   One-to-one relations use a single optional foreign key field with @relation specifying fields and references.

2. Step 2: Verify the syntax

   The correct schema declares profile Profile? @relation(fields: [profileId], references: [id]) and profileId Int? in User, with user User? in Profile. This matches the pattern for optional one-to-one.

3. Final Answer:

   Schema with explicit optional FK and @relation -> Option B

4. Quick Check:

   One-to-one uses single foreign key with @relation [OK]

Hint: One-to-one needs single foreign key with @relation fields [OK]

Common Mistakes:

• Using array [] for one-to-one relation
• Missing @relation fields and references
• Not making foreign key optional

3. Given the Prisma models below, what will be the result of the query prisma.user.findUnique({ where: { id: 1 }, include: { posts: true } }) if user with id 1 has two posts?

model User {
  id    Int    @id @default(autoincrement())
  name  String
  posts Post[]
}

model Post {
  id      Int   @id @default(autoincrement())
  title   String
  userId  Int
  user    User  @relation(fields: [userId], references: [id])
}

medium

A. An error because 'include' is not valid here

B. An object with user data but 'posts' will be undefined

C. An object with user data and an array of two post objects under 'posts'

D. An array of posts only without user data

Solution

1. Step 1: Understand the Prisma query with include

   The query fetches a user by id and includes related posts as an array.

2. Step 2: Check relation and data

   User with id 1 has two posts, so 'posts' will be an array of two post objects inside the user object.

3. Final Answer:

   User object with posts array of two posts -> Option C

4. Quick Check:

   Include fetches related data as array [OK]

Hint: Include fetches related records as array or object [OK]

Common Mistakes:

• Expecting posts to be undefined without include
• Thinking include causes error
• Expecting only posts without user

4. Identify the error in this Prisma schema for a one-to-many relation between Author and Book models:

model Author {
  id    Int    @id @default(autoincrement())
  name  String
  books Book
}

model Book {
  id       Int    @id @default(autoincrement())
  title    String
  authorId Int
  author   Author @relation(fields: [authorId], references: [id])
}

medium

A. Author model is missing a primary key

B. Missing @relation on Author.books field

C. authorId should be optional in Book model

D. The 'books' field in Author should be an array (Book[]) not a single Book

Solution

1. Step 1: Check relation type from schema

   Author to Book is one-to-many, so Author.books must be an array of Book.

2. Step 2: Identify the mistake

   Author.books is declared as a single Book, which is incorrect for one-to-many.

3. Final Answer:

   'books' must be Book[] array -> Option D

4. Quick Check:

   One-to-many uses array field [OK]

Hint: One-to-many needs array [] on 'many' side field [OK]

Common Mistakes:

• Using single object instead of array for many side
• Forgetting @relation decorator
• Making foreign key optional incorrectly

5. You want to fetch all Order records with their related Customer data, but only include customers who have placed more than 5 orders. Which Prisma query approach correctly applies this filter using relations?

model Customer {
  id     Int     @id @default(autoincrement())
  name   String
  orders Order[]
}

model Order {
  id         Int      @id @default(autoincrement())
  total      Float
  customerId Int
  customer   Customer @relation(fields: [customerId], references: [id])
}

hard

A. Use findMany on Order with include: { customer: true } and a where on customer orders count > 5

B. Use findMany on Customer with where: { orders: { some: {} } } and include orders

C. Use findMany on Order with where: { customer: { orders: { count: { gt: 5 } } } } and include customer

D. Use findMany on Customer with include: { orders: true } and filter orders count > 5 in JavaScript

Solution

1. Step 1: Understand filtering by related model count

   Prisma allows filtering with nested conditions on related models using 'where' and relation filters.

2. Step 2: Identify correct query

   Using findMany on Order with include: { customer: true } and where: { customer: { orders: { _count: { gt: 5 } } } } fetches the desired orders with customers who have more than 5 orders.

3. Final Answer:

   findMany on Order with relational where filter and customer include -> Option A

4. Quick Check:

   Filter related model count in where clause [OK]

Hint: Filter related model count in where clause with include [OK]

Common Mistakes:

• Filtering in JavaScript instead of query
• Using incorrect nested where syntax
• Filtering on Customer without including orders count