Relationship design patterns in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a one-to-one relationship in database design?

A one-to-one relationship means each record in one table is linked to exactly one record in another table. It's like having a unique ID card for each person.

beginner

Explain a one-to-many relationship with an example.

In a one-to-many relationship, one record in a table can be linked to many records in another table. For example, one author can write many books.

intermediate

What is a many-to-many relationship and how is it usually implemented?

Many-to-many means multiple records in one table relate to multiple records in another. It's often done using a join table that connects the two tables.

intermediate

How does a GraphQL schema represent a one-to-many relationship?

GraphQL uses fields that return lists of related objects. For example, a type Author can have a field books that returns a list of Book objects.

beginner

Why is it important to design relationships carefully in databases?

Good relationship design helps keep data organized, avoids duplication, and makes queries faster and easier to write.

Which relationship pattern allows multiple records in one table to relate to multiple records in another?

A One-to-one

B One-to-many

C Many-to-many

D None of the above

Many-to-many relationships connect multiple records from both tables, often using a join table.

In GraphQL, how do you represent a one-to-many relationship?

A Using a field that returns a single object

B Using scalar types only

C Using enums

D Using a field that returns a list of objects

A one-to-many relationship is represented by a field returning a list of related objects.

What is the purpose of a join table in many-to-many relationships?

A To store unrelated data

B To connect records from two tables

C To speed up queries by caching

D To store user permissions

Join tables link records from two tables in many-to-many relationships.

Which of these is NOT a common relationship pattern in databases?

A Many-to-zero

B One-to-many

C Many-to-many

D One-to-one

Many-to-zero is not a recognized relationship pattern.

Why should relationships be designed carefully in databases?

A To avoid data duplication and improve query speed

B To make data messy

C To confuse users

D To increase storage space

Careful design avoids duplication and makes data easier to manage and query.

Describe the three main types of relationship design patterns in databases and give a simple example for each.

Think about how many records in one table connect to records in another.

Explain how you would represent a many-to-many relationship in a GraphQL schema.

Consider how GraphQL fields can return lists to show multiple connections.

Practice

1. In GraphQL, which relationship pattern describes one item connected to many items, like one author having many books?

easy

A. One-to-many

B. Many-to-one

C. Many-to-many

D. One-to-one

Solution

1. Step 1: Understand relationship types

   One-to-many means one record links to many records, like one author to many books.

2. Step 2: Match example to pattern

   Since one author can have many books, this fits one-to-many.

3. Final Answer:

   One-to-many -> Option A

4. Quick Check:

   One author -> many books = One-to-many [OK]

Hint: One item linked to many items means one-to-many [OK]

Common Mistakes:

• Confusing many-to-many with one-to-many
• Thinking one-to-one fits multiple items
• Mixing up direction of relationship

2. Which of the following is the correct GraphQL syntax to define a many-to-many relationship between Student and Course types?

easy

A. type Student { courses: Course }
type Course { students: Student }

B. type Student { course: Course }
type Course { student: Student }

C. type Student { courses: [Course!]! }
type Course { students: [Student!]! }

D. type Student { courses: Course! }
type Course { students: Student! }

Solution

1. Step 1: Identify many-to-many syntax

   Many-to-many uses lists on both sides, so fields must be arrays like [Course!]! and [Student!]!.

2. Step 2: Check options for correct list syntax

   type Student { courses: [Course!]! } type Course { students: [Student!]! } uses lists with non-nullable items and non-nullable lists, which is correct GraphQL syntax.

3. Final Answer:

   type Student { courses: [Course!]! } type Course { students: [Student!]! } -> Option C

4. Quick Check:

   Lists on both sides = many-to-many syntax [OK]

Hint: Many-to-many uses lists on both types in GraphQL [OK]

Common Mistakes:

• Using single objects instead of lists for many-to-many
• Missing non-nullable markers (!)
• Confusing one-to-one syntax with many-to-many

3. Given this GraphQL schema snippet:

type Author {
  id: ID!
  books: [Book!]!
}

type Book {
  id: ID!
  title: String!
  author: Author!
}

What will the query { author(id: "1") { books { title } } } return if author 1 has two books titled "A" and "B"?

medium

A. {"data":{"author":{"books":{"title":"A"}}}}

B. {"data":{"author":{"books":[{"title":"A"},{"title":"B"}]}}}

C. {"data":{"author":{"books":null}}}

D. {"errors":[{"message":"Field 'books' not found"}]}

Solution

1. Step 1: Understand schema and query

   Author has a list of books. Query asks for author with id "1" and their books' titles.

2. Step 2: Predict query result

   Since author 1 has two books "A" and "B", the books field returns a list with both titles.

3. Final Answer:

   {"data":{"author":{"books":[{"title":"A"},{"title":"B"}]}}} -> Option B

4. Quick Check:

   List of books with titles A and B returned [OK]

Hint: List fields return arrays of objects in GraphQL query results [OK]

Common Mistakes:

• Expecting a single book object instead of a list
• Assuming null if multiple books exist
• Confusing error with missing field

4. Consider this GraphQL type definition:

type User {
  id: ID!
  posts: Post
}

type Post {
  id: ID!
  content: String!
}

What is the main problem with this design if a user can have many posts?

medium

A. The 'posts' field should be a list, not a single Post object.

B. The 'id' field is missing in User type.

C. The 'content' field should be nullable.

D. The Post type should include a reference back to User.

Solution

1. Step 1: Analyze the 'posts' field type

   The 'posts' field is declared as a single Post, but users can have many posts.

2. Step 2: Identify correct type for multiple posts

   To represent many posts, 'posts' should be a list like [Post!]! instead of a single Post.

3. Final Answer:

   The 'posts' field should be a list, not a single Post object. -> Option A

4. Quick Check:

   Multiple posts need list type, not single object [OK]

Hint: Multiple related items need list type in GraphQL [OK]

Common Mistakes:

• Thinking single object can hold many items
• Ignoring list syntax for multiple relations
• Assuming missing fields cause this error

5. You want to model a social network in GraphQL where users can follow many other users and be followed by many users. Which relationship design pattern and schema snippet correctly represents this many-to-many self-relationship?

hard

A. type User { follower: User following: User } // single user on each side

B. type User { followers: [User] following: User } // mixed list and single user

C. type User { followers: User! following: User! } // non-null single user

D. type User { followers: [User!]! following: [User!]! } // both lists represent many-to-many

Solution

1. Step 1: Understand many-to-many self-relationship

   Users can follow many users and be followed by many users, so both sides need lists.

2. Step 2: Check schema for correct list usage

   type User { followers: [User!]! following: [User!]! } // both lists represent many-to-many uses lists for both followers and following, correctly modeling many-to-many self-relationship.

3. Final Answer:

   type User { followers: [User!]! following: [User!]! } -> Option D

4. Quick Check:

   Lists on both sides model many-to-many self-relationship [OK]

Hint: Self many-to-many needs lists on both fields [OK]

Common Mistakes:

• Using single User instead of list for followers/following
• Mixing list and single object types
• Forgetting non-nullable list syntax