There are a lot of different kinds of errors in GraphQL! In this guide, we’ll discuss some of the main categories and learn when they apply.
Because GraphQL is strongly typed, it performs validation of all queries before executing them. If an incoming query is invalid, it isn’t executed. Instead, a response is sent back with "errors":
{
"errors" => [ ... ]
}
Each error has a message, line, column and path.
The validation rules are part of the GraphQL specification and built into GraphQL-Ruby, so there’s not really a way to customize this behavior, except to pass validate: false when executing a query, which skips validation altogether.
GraphQL-Ruby supports pre-execution analysis, which may return "errors" instead of running a query. You can find details in the Analysis guide.
While GraphQL-Ruby is executing a query, some constraints must be satisfied. For example:
nil.These constraints are part of the GraphQL specification, and when they are violated, it must be addressed somehow. Read more in Type Errors.
"errors"The GraphQL specification provides for a top-level "errors" key which may include information about errors during query execution. "errors" and "data" may both be present in the case of a partial success.
In your own schema, you can add to the "errors" key by raising GraphQL::ExecutionError (or subclasses of it) in your code. Read more in the Execution Errors guide.
A schema can be configured to handle certain errors during field execution with handlers that you give it, using rescue_from. Read more in the Error Handling guide.
When a raised error is not rescued, the GraphQL query crashes entirely and the surrounding code (like a Rails controller) must handle the exception.
For example, Rails will probably return a generic 500 page.
When you want end users (human beings) to read error messages, you can express errors in the schema, using normal GraphQL fields and types. In this approach, errors are strongly-typed data, queryable in the schema, like any other application data.
For more about this approach, see Mutation Errors