Field-level errors in GraphQL - Step-by-Step Execution

Concept Flow - Field-level errors

Client sends GraphQL query

↓

Server processes query

↓

Resolve each field

↓

Field OK

↓

Include data

↓

Build response with data and errors

↓

Send response to client

The server processes each field in the query separately, collecting data or errors per field, then sends a combined response with both data and field-level errors.

Execution Sample

GraphQL

query {
  user(id: "1") {
    name
    email
    posts {
      title
    }
  }
}

A query requesting user name, email, and post titles; some fields may return errors individually.

Execution Table

Step	Field	Action	Result	Error Included
1	user	Resolve user by id	User object found	No
2	name	Resolve name field	"Alice"	No
3	email	Resolve email field	null	Yes: Email not available
4	posts	Resolve posts field	List of posts	No
5	posts.title	Resolve title for each post	["Post 1", "Post 2"]	No
6	Build response	Combine data and errors	Data with null email and error message	Yes
7	Send response	Return to client	Response with data and field-level error	Yes

💡 All fields processed; errors included only for fields that failed

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
data	{}	{"user": {}}	{"user": {"name": "Alice"}}	{"user": {"name": "Alice", "email": null}}	{"user": {"name": "Alice", "email": null, "posts": []}}	{"user": {"name": "Alice", "email": null, "posts": [{"title": "Post 1"}, {"title": "Post 2"}]}}	{"user": {"name": "Alice", "email": null, "posts": [{"title": "Post 1"}, {"title": "Post 2"}]}}
errors	[]	[]	[]	[{"field": "email", "message": "Email not available"}]	[{"field": "email", "message": "Email not available"}]	[{"field": "email", "message": "Email not available"}]	[{"field": "email", "message": "Email not available"}]

Key Moments - 2 Insights

Why does the 'email' field have a null value but also an error message?

Because the server returns partial data for the user, setting 'email' to null when it cannot resolve it, and includes an error message specifically for that field as shown in step 3 and 6 of the execution_table.

Does an error in one field stop the entire query from returning data?

No, the server resolves each field independently, so errors in one field (like 'email') do not prevent other fields (like 'name' and 'posts') from returning data, as seen in steps 2, 3, and 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'email' after step 3?

A "[e-mail]"

B null

C "unknown"

D undefined

💡 Hint

Check the 'Result' column for step 3 in the execution_table.

At which step does the server include an error for the 'email' field?

A Step 3

B Step 2

C Step 5

D Step 7

💡 Hint

Look at the 'Error Included' column in the execution_table for when 'email' has an error.

If the 'posts' field failed to resolve, what would change in the execution_table?

A Only the 'name' field would have an error

B The entire query would fail and return no data

C The 'posts' field would have an error and data would be null for posts

D No errors would be included

💡 Hint

Refer to how errors are handled per field in the execution_table and concept_flow.

Concept Snapshot

Field-level errors in GraphQL mean each field is resolved separately.
If a field fails, its value is null and an error message is included.
Other fields still return data normally.
The response contains both 'data' and 'errors' sections.
This allows partial success with detailed error info.

Full Transcript

In GraphQL, when a client sends a query, the server processes each requested field one by one. If a field resolves successfully, its data is included in the response. If a field fails, the server sets that field's value to null and adds an error message about that field. This way, the client receives as much data as possible along with clear information about which fields had problems. For example, if the 'email' field cannot be retrieved, it will be null in the data, and an error will explain why. Other fields like 'name' and 'posts' still return their data. The final response combines all this information, helping clients handle partial failures gracefully.

Practice

1. What is the main purpose of field-level errors in GraphQL queries?

easy

A. To show exactly which field in the query caused an error

B. To speed up the query execution time

C. To encrypt the query data for security

D. To automatically fix errors in the query

Solution

1. Step 1: Understand what field-level errors represent

   Field-level errors identify the specific part of a query that failed, not the whole query.

2. Step 2: Recognize the benefit of field-level errors

   This helps apps show clear messages next to the exact problem fields, improving user experience.

3. Final Answer:

   To show exactly which field in the query caused an error -> Option A

4. Quick Check:

   Field-level errors = pinpoint error location [OK]

Hint: Field-level errors pinpoint the exact failing field [OK]

Common Mistakes:

• Thinking errors speed up queries
• Believing errors fix themselves
• Confusing errors with encryption

2. Which of the following is the correct way to represent a field-level error path in a GraphQL error response?

easy

A. "location": ["user", "email"]

B. "path": "user.email"

C. "error": "user.email"

D. "path": ["user", "email"]

Solution

1. Step 1: Identify the standard format for error paths

   GraphQL error paths are arrays showing the nested fields causing the error, e.g., ["user", "email"].

2. Step 2: Compare options to the standard format

   "path": ["user", "email"] matches the array format; others use strings or wrong keys.

3. Final Answer:

   "path": ["user", "email"] -> Option D

4. Quick Check:

   Error path = array of field names [OK]

Hint: Error paths use arrays, not dot strings [OK]

Common Mistakes:

• Using dot notation strings for paths
• Confusing 'error' with 'path' key
• Using 'location' instead of 'path'

3. Given this GraphQL error response snippet:

{"errors": [{"message": "Cannot query field 'age' on type 'User'", "path": ["user", "age"]}]}

What does the path ["user", "age"] indicate?

medium

A. The error is in the 'age' field inside the 'user' object

B. The entire 'user' object is missing

C. The query has a syntax error at the root

D. The 'age' field is null but valid

Solution

1. Step 1: Analyze the error message and path

   The message says 'Cannot query field age on type User', and the path shows the nested location: user -> age.

2. Step 2: Interpret the meaning of the path

   This means the error is specifically with the 'age' field inside the 'user' object, not the whole user or root.

3. Final Answer:

   The error is in the 'age' field inside the 'user' object -> Option A

4. Quick Check:

   Path shows exact field with error [OK]

Hint: Error path shows nested field causing issue [OK]

Common Mistakes:

• Assuming whole object is missing
• Confusing syntax errors with field errors
• Thinking null fields cause this error

4. You receive this GraphQL error response:

{"errors": [{"message": "Field 'price' must be a positive number", "path": ["product", "price"]}]}

Which fix will correctly address the field-level error?

medium

A. Ignore the error and retry the query

B. Change the 'price' value to a positive number in the query variables

C. Rename 'price' to 'cost' in the query

D. Remove the 'price' field from the query entirely

Solution

1. Step 1: Understand the error message and path

   The error says 'price' must be positive, and the path points to product -> price field.

2. Step 2: Determine the correct fix

   Fixing means providing a positive number for 'price' in the query variables, not removing or renaming the field.

3. Final Answer:

   Change the 'price' value to a positive number in the query variables -> Option B

4. Quick Check:

   Fix value to meet field rules [OK]

Hint: Fix field value to meet error message rules [OK]

Common Mistakes:

• Removing required fields
• Renaming fields without schema support
• Ignoring errors expecting auto-fix

5. You have a GraphQL query requesting nested fields:

{ user { name, posts { title, comments { text, author } } } }

If the error response shows:

{"errors": [{"message": "Cannot query field 'author' on type 'Comment'", "path": ["user", "posts", 0, "comments", 2, "author"]}]}

What does this error tell you about the query and how should you fix it?

hard

A. The 'comments' array is empty; add comments to fix the error

B. The 'posts' field is missing; add it to the query

C. The 'author' field does not exist on 'Comment'; remove or correct it in the query

D. The 'user' field is invalid; rename it to 'member'

Solution

1. Step 1: Analyze the error message and path

   The error says 'author' is not a valid field on 'Comment'. The path shows it is nested inside user -> posts -> comments at index 2.

2. Step 2: Decide the fix based on schema rules

   Since 'author' is invalid, you should remove or correct this field in the query to match the schema.

3. Final Answer:

   The 'author' field does not exist on 'Comment'; remove or correct it in the query -> Option C

4. Quick Check:

   Field error means fix or remove invalid field [OK]

Hint: Invalid field in nested path means fix query field name [OK]

Common Mistakes:

• Assuming empty arrays cause this error
• Adding missing fields not in schema
• Renaming unrelated fields