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.