One-to-one relationships in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Querying a one-to-one relationship in GraphQL

Given the following GraphQL schema snippet:

type User {
  id: ID!
  name: String!
  profile: Profile
}

type Profile {
  id: ID!
  bio: String
  user: User!
}

What will be the result of this query?

{
  user(id: "1") {
    name
    profile {
      bio
    }
  }
}

A {"data": {"user": null}}

B {"data": {"user": {"name": "Alice", "profile": {"bio": "Loves hiking"}}}}

C {"data": {"user": {"name": "Alice", "profile": null}}}

D SyntaxError: Query missing required fields

💡 Hint

Remember that in a one-to-one relationship, the related object is either present or null.

The query asks for a user with id "1" and their profile's bio. Since the user has a profile, the bio is returned.

🧠 Conceptual

intermediate

Understanding one-to-one relationships in GraphQL

Which statement best describes a one-to-one relationship in GraphQL?

A Each object in type A is linked to exactly one object in type B, and vice versa.

B Each object in type A can be linked to many objects in type B.

C Objects in type A and B are unrelated and cannot reference each other.

D Each object in type A is linked to zero or more objects in type B.

💡 Hint

Think about the meaning of 'one-to-one' in everyday relationships.

One-to-one means each object on one side matches exactly one object on the other side.

📝 Syntax

advanced

Identify the syntax error in this GraphQL schema for one-to-one

Find the syntax error in this GraphQL schema snippet defining a one-to-one relationship:

type Author {
  id: ID!
  name: String!
  biography: Biography
}

type Biography {
  id: ID!
  text: String!
  author: Author
}

A There is no syntax error; the schema is valid.

B The field 'author' in Biography should be non-nullable (Author!).

C The type 'Biography' must have a list of authors, not a single author.

D The field 'biography' in Author should be non-nullable (Biography!).

💡 Hint

Check if the schema syntax follows GraphQL rules for types and fields.

The schema is syntactically valid. Nullability does not cause syntax errors.

❓ optimization

advanced

Optimizing data fetching in one-to-one GraphQL queries

You want to fetch a user and their profile data in one query. Which approach is best to avoid unnecessary data fetching?

A Query only user fields and fetch profile data later with a separate query.

B Query user and profile fields separately in two queries.

C Query user with profile fields nested in the same query.

D Query profile fields only and ignore user fields.

💡 Hint

Think about how GraphQL allows nested queries to reduce round trips.

Querying nested fields in one query reduces network calls and improves performance.

🔧 Debug

expert

Debugging null profile in one-to-one GraphQL query

Given this query:

{
  user(id: "2") {
    name
    profile {
      bio
    }
  }
}

The response is:

{"data": {"user": {"name": "Bob", "profile": null}}}

What is the most likely cause?

A The query syntax is invalid and causes profile to be null.

B The profile field is not defined in the GraphQL schema.

C The user with id "2" does not exist.

D User with id "2" does not have a linked profile in the database.

💡 Hint

Consider what a null nested object means in GraphQL responses.

A null nested object means the relation exists but no linked data is found.

Practice

1. What does a one-to-one relationship in GraphQL represent?

easy

A. Each item in one type is linked to exactly one item in another type.

B. Each item in one type can link to many items in another type.

C. Items are not linked between types.

D. One item links to zero or many items in the same type.

Solution

1. Step 1: Understand relationship types

   One-to-one means each item in one set matches exactly one item in another set.

2. Step 2: Apply to GraphQL context

   In GraphQL, this means a field in one type points to exactly one item in another type.

3. Final Answer:

   Each item in one type is linked to exactly one item in another type. -> Option A

4. Quick Check:

   One-to-one = single matching pair [OK]

Hint: One-to-one means exactly one match per item [OK]

Common Mistakes:

• Confusing one-to-one with one-to-many
• Thinking items can link to multiple items
• Assuming no links exist

2. Which of the following is the correct GraphQL syntax to define a one-to-one relationship between User and Profile types?

easy

A. type User { profile: String }

B. type User { profiles: [Profile] }

C. type User { profile: [Profile] }

D. type User { profile: Profile }

Solution

1. Step 1: Identify one-to-one field type

   One-to-one means a single object, so the field should be of type Profile, not a list.

2. Step 2: Check syntax correctness

   type User { profile: Profile } uses profile: Profile, which is correct syntax for one-to-one.

3. Final Answer:

   type User { profile: Profile } -> Option D

4. Quick Check:

   Single object field = one-to-one [OK]

Hint: Use single type, not list, for one-to-one fields [OK]

Common Mistakes:

• Using list syntax for one-to-one
• Using wrong field type like String
• Missing colon or braces

3. Given the schema:

type User { id: ID! name: String profile: Profile }

type Profile { id: ID! bio: String }

And the query:

{ user(id: "1") { name profile { bio } } }

If user with id "1" has name "Alice" and profile bio "Loves cats", what is the expected output?

medium

A. { "data": { "user": { "name": "Alice", "profile": { "bio": "Loves cats" } } } }

B. { "data": { "user": { "name": "Alice", "profile": null } } }

C. { "data": { "user": { "name": "Alice" } } }

D. Error: profile field missing

Solution

1. Step 1: Understand query fields

   The query requests user's name and profile's bio.

2. Step 2: Match data to query

   User "Alice" has a profile with bio "Loves cats", so both fields return data.

3. Final Answer:

   { "data": { "user": { "name": "Alice", "profile": { "bio": "Loves cats" } } } } -> Option A

4. Quick Check:

   Query fields match data output [OK]

Hint: Returned data matches requested fields exactly [OK]

Common Mistakes:

• Assuming profile is null when data exists
• Omitting nested profile field
• Expecting error when schema is correct

4. Identify the error in this GraphQL schema for one-to-one relationship:

type User { id: ID! profile: [Profile] }

type Profile { id: ID! user: User }

medium

A. User type must not have profile field.

B. Using list syntax [Profile] for one-to-one relationship is incorrect.

C. Missing exclamation mark after Profile.

D. Profile type cannot reference User.

Solution

1. Step 1: Check field type for one-to-one

   One-to-one means a single object, not a list. Using [Profile] means many profiles.

2. Step 2: Confirm correct syntax

   Field should be profile: Profile without brackets for one-to-one.

3. Final Answer:

   Using list syntax [Profile] for one-to-one relationship is incorrect. -> Option B

4. Quick Check:

   List means many, not one [OK]

Hint: No brackets for one-to-one fields [OK]

Common Mistakes:

• Confusing one-to-one with one-to-many
• Adding unnecessary exclamation marks
• Thinking references are not allowed

5. You want to model a one-to-one relationship between Author and Biography in GraphQL. Which schema design ensures each author has exactly one biography and each biography belongs to exactly one author?

hard

A. type Author { id: ID! name: String biography: Biography } type Biography { id: ID! text: String }

B. type Author { id: ID! name: String biographies: [Biography]! } type Biography { id: ID! text: String author: Author }

C. type Author { id: ID! name: String biography: Biography! } type Biography { id: ID! text: String author: Author! }

D. type Author { id: ID! name: String biographyId: ID! } type Biography { id: ID! text: String }

Solution

1. Step 1: Check for required one-to-one fields

   Both Author and Biography have non-nullable fields pointing to each other, ensuring exactly one match.

2. Step 2: Validate mutual references

   Each Author has one Biography! and each Biography has one Author!, enforcing one-to-one.

3. Final Answer:

   type Author { id: ID! name: String biography: Biography! } type Biography { id: ID! text: String author: Author! } -> Option C

4. Quick Check:

   Mutual non-null fields = strict one-to-one [OK]

Hint: Use non-null mutual fields for strict one-to-one [OK]

Common Mistakes:

• Using lists instead of single objects
• Making fields nullable allowing missing links
• Not linking both types mutually