customvalidation.rst

Implementing custom validators

GraphQL uses query validators to check if Query AST is valid and can be executed. Every GraphQL server implements standard query validators.

For example, a validator tests if a queried field exists on queried type, making the query fail with a "Cannot query field on type" error if it does not.

If you need more complex validation than presented before, you can implement your own query validators.

Implementing your custom validators

All custom query validators should extend the ValidationRule base class importable from the graphql.validation.rules module. Query validators are Visitor classes.

Your custom validator should implement some methods of

In order to perform validation, your validator class should define some one or more of enter_* or leave_* methods.

Foreach methods, you will receive a node object to test:

• You can now choose to raise an error by calling report_error method with an instance of GraphQLError describing the failure reason.
• Or you can continue the parsing by returning None.

Example

Here is an example query validator that only allows queries fields with a name of even length.

from graphql import GraphQLError
from graphql.language import FieldNode
from graphql.validation import ValidationRule


class MyCustomValidationRule(ValidationRule):
    def enter_field(self, node: FieldNode, *_args):
        if len(node.name.value) % 2 == 0:
            # Here the query length is even, so we allow it.
            return
        else:
            # Here the query length is odd, so we don't want to allow it.
            # Calling self.report_error will make the query fail with the error message.
            self.report_error(
                GraphQLError(
                    f"Cannot query '{field_name}': length is odd.", node,
                )
            )