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Type Resolvers in RESTHeart GraphQL

Custom resolvers provide advanced flexibility in how GraphQL fields are resolved in RESTHeart. This guide covers resolver types, implementation patterns, and best practices.

Understanding Resolvers

In RESTHeart GraphQL, Type resolvers determine concrete types for interfaces and unions

Interface Type Resolvers

Type resolvers for interfaces determine which concrete type a document represents:

{
    "mappings": {
        "Content": {
            "$typeResolver": {
                "Article": "field-exists(wordCount)",
                "Video": "field-exists(duration)",
                "Podcast": "field-exists(audioUrl)"
            }
        }
    }
}

Union Type Resolvers

Similar to interfaces, but for union types:

{
    "mappings": {
        "SearchResult": {
            "$typeResolver": {
                "User": "field-exists(email)",
                "Product": "field-exists(price)",
                "Article": "field-exists(content)"
            }
        }
    }
}

Type Resolution Rules

  1. Evaluation Order

    • Rules are evaluated in order

    • First matching rule determines the type

    • Include a default case when possible

  2. Available Functions

    • field-exists(fieldName): Checks if a field exists

    • field-equals(fieldName, value): Compares field value

    • field-type(fieldName, type): Checks field data type

{
    "mappings": {
        "Node": {
            "$typeResolver": {
                "Document": "field-type(content, string)",
                "Image": "field-equals(type, 'image')",
                "Default": "true"
            }
        }
    }
}