One-to-one relationships in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a one-to-one relationship in databases?

A one-to-one relationship means each record in one table is linked to exactly one record in another table, and vice versa.

beginner

How do you represent a one-to-one relationship in GraphQL schema?

You use fields in types that reference each other, often with unique IDs to link exactly one record from one type to one record in another.

intermediate

Why use one-to-one relationships instead of combining data in one type?

To keep data organized, avoid duplication, and separate concerns, especially when some data is optional or sensitive.

intermediate

In GraphQL, how can you ensure a one-to-one relationship is enforced?

By using unique identifiers and resolvers that fetch exactly one related record, and by database constraints like unique foreign keys.

beginner

Give a simple example of a one-to-one relationship in GraphQL types.

type User { id: ID! profile: Profile } type Profile { id: ID! user: User } Each User has one Profile and each Profile belongs to one User.

What does a one-to-one relationship mean?

A One record links to many records in another table

B Many records link to many records in another table

C Each record in one table links to exactly one record in another table

D No records are linked between tables

A one-to-one relationship means each record in one table is linked to exactly one record in another table.

In GraphQL, how do you link two types in a one-to-one relationship?

A By using arrays to hold multiple records

B By merging both types into one

C By ignoring relationships

D By adding a field in one type that references the other type

You link two types by adding a field in one type that references the other type.

Why might you separate data into two types with a one-to-one relationship?

A To make queries slower

B To organize data and keep sensitive info separate

C To duplicate data unnecessarily

D To avoid using IDs

Separating data helps organize it and keeps sensitive or optional info separate.

Which database constraint helps enforce one-to-one relationships?

A Unique foreign key

B Non-unique index

C Primary key on both tables

D No constraints needed

A unique foreign key ensures each record links to only one record in the related table.

In a GraphQL one-to-one example, what does the User type typically contain?

A A field referencing Profile

B An array of Profiles

C No fields referencing other types

D Multiple unrelated fields

User type contains a field referencing Profile to show the one-to-one link.

Explain what a one-to-one relationship is and how you would model it in GraphQL.

Think about how each record links to exactly one record in another type.

Describe why one-to-one relationships are useful and give a simple GraphQL example.

Consider when you want to keep some data separate but connected.

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