One-to-one relationships in GraphQL - Step-by-Step Execution

Concept Flow - One-to-one relationships

Define Type A

↓

Define Type B

↓

Add field in Type A referencing Type B

↓

Add field in Type B referencing Type A

↓

Query Type A with linked Type B

↓

Return combined data from both types

This flow shows how two GraphQL types are linked one-to-one by referencing each other, then queried together.

Execution Sample

GraphQL

type User {
  id: ID!
  profile: Profile
}

type Profile {
  id: ID!
  user: User
}

query {
  user(id: "1") {
    id
    profile {
      id
    }
  }
}

Defines User and Profile types linked one-to-one, then queries a user with their profile.

Execution Table

Step	Action	Evaluation	Result
1	Define User type with profile field	User has profile field of type Profile	User and Profile types linked
2	Define Profile type with user field	Profile has user field of type User	Bidirectional link established
3	Query user with id '1'	Find user with id '1'	User object found
4	Resolve profile field for user	Fetch profile linked to user '1'	Profile object with id '101' found
5	Return user data with profile id	Combine user and profile data	{ user: { id: "1", profile: { id: "101" } } }
6	End of query execution	All requested fields resolved	Query complete

💡 Query ends after all requested fields are resolved and data returned

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
user	undefined	{ id: "1" }	{ id: "1", profile: { id: "101" } }	{ id: "1", profile: { id: "101" } }
profile	undefined	undefined	{ id: "101" }	{ id: "101" }

Key Moments - 2 Insights

Why do both User and Profile types reference each other?

Because in a one-to-one relationship, each side holds a reference to the other to allow querying from either type, as shown in execution_table steps 1 and 2.

What happens if the profile field is missing for a user?

The profile field would be null or undefined in the result, meaning no linked profile exists, as would be seen if step 4 returned no profile.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'profile' after step 4?

A { id: '101' }

B undefined

C { id: '1' }

D null

💡 Hint

Check the 'profile' variable value in variable_tracker after step 4

At which step is the user with id '1' found?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Action' and 'Evaluation' columns in execution_table for user lookup

If the profile field was removed from User type, what would happen in the query?

A Query would fail at step 3

B Profile data would not be returned

C User data would be missing

D Query would return an error at step 5

💡 Hint

Consider how the profile field is resolved in step 4 and returned in step 5

Concept Snapshot

One-to-one relationships in GraphQL:
- Define two types referencing each other
- Each type has a field linking to the other
- Query one type and include the linked type's fields
- Result returns combined data from both linked types
- Enables easy navigation between related single objects

Full Transcript

This visual execution shows how one-to-one relationships work in GraphQL. First, two types, User and Profile, are defined with fields referencing each other. This creates a bidirectional link. Then a query requests a user by id and includes the user's profile data. The execution steps find the user, then resolve the linked profile, and finally return combined data. Variables track the user and profile objects as they are fetched. Key moments clarify why both types reference each other and what happens if linked data is missing. Quiz questions test understanding of variable values and query steps. The snapshot summarizes the key points for quick reference.