hasura/graphql-engine · error · TypeMappingCollectionError

Cannot return a predicate type from a command

Error message

Cannot return a predicate type from a command

What it means

A command declares a predicate type as (part of) its output. Predicates are filter inputs and cannot be returned from commands; the resolver rejects this during type mapping collection.

Source

Thrown at v3/crates/metadata-resolve/src/helpers/type_mappings.rs:45

    #[error(
        "No support for using the same type {type_name:} against multiple data connector objects {ndc_type_1:} and {ndc_type_2:}"
    )]
    MappingToMultipleDataConnectorObjectType {
        type_name: Qualified<CustomTypeName>,
        ndc_type_1: DataConnectorObjectType,
        ndc_type_2: DataConnectorObjectType,
    },
    #[error(
        "Missing mapping for field {field_name:} when mapping type {type_name:} to object {ndc_type_name:} of data connector {data_connector:}"
    )]
    MissingFieldMapping {
        type_name: Qualified<CustomTypeName>,
        field_name: FieldName,
        data_connector: Qualified<DataConnectorName>,
        ndc_type_name: DataConnectorObjectType,
    },

    #[error("Cannot return a predicate type from a command")]
    PredicateAsResponseType,

    #[error("Internal Error: Unknown type {type_name:} when collecting type mappings")]
    InternalUnknownType {
        type_name: Qualified<CustomTypeName>,
    },
    #[error("ndc validation error: {0}")]
    NDCValidationError(#[from] NDCValidationError),
}

// Special case handling for commands with response config; as they don't match
// the ndc type mapping exactly.
#[derive(Debug)]
pub(crate) struct SpecialCaseTypeMapping<'a> {
    pub(crate) response_config: &'a CommandsResponseConfig,
    pub(crate) ndc_object_type: &'a ndc_models::ObjectType,
}

View on GitHub (pinned to 724551b9ae)

Solutions

1. Change the command's output type to a non-predicate object or scalar type
2. If filtering behavior is intended, model it as an argument rather than the output

fn is_predicate_type(t: &QualifiedTypeName) -> bool { /* check against known predicate types */ true }

When it happens

Trigger: Setting a command's output type to a predicate/boolean-expression type; mistakenly reusing an input predicate type as a response type.

Common situations: Confusing filter input types with response types; copy-paste from argument definitions.

Related errors

• mapping for type {type_name} of command {command_name} is no
• cannot get absolute path: %w
• the type {unknown_ndc_type:} is not defined as an object typ
• function {function_name} is not defined in data connector {d
• column {column_name} is not defined in function/procedure {f

AI-assisted analysis of hasura/graphql-engine@724551b9ae (2026-08-28). Data as JSON: /api/errors/3aa8eb9434292427. Report an issue: GitHub.