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SchemaUnions

Union Types

TypeScript union types of object types are converted to GraphQL union types.

Basic Usage

import type { Post } from "./Post";
import type { Comment } from "./Comment";
 
/**
 * Content that can be searched
 */
export type SearchResult = Post | Comment;

Generates:

"""Content that can be searched"""
union SearchResult = Comment | Post

Using Unions in Resolvers

import { defineQuery } from "../gqlkit";
import type { SearchResult } from "./types";
 
export const search = defineQuery<{ query: string }, SearchResult[]>(
  (_root, args, ctx) => {
    return ctx.db.search(args.query);
  }
);

Generates:

type Query {
  search(query: String!): [SearchResult!]!
}

Inline Unions

When a field type is an inline union of object types, gqlkit automatically generates a GraphQL Union type. The generated type name follows the convention {ParentTypeName}{PascalCaseFieldName}:

import type { User } from "./User";
import type { Post } from "./Post";
 
export type SearchResult = {
  id: string;
  /**
   * The matched item - either a User or a Post
   */
  item: User | Post;
};

Generates:

type SearchResult {
  id: String!
  """The matched item - either a User or a Post"""
  item: SearchResultItem!
}
 
union SearchResultItem = Post | User

Nullable Inline Unions

Inline unions can be nullable:

export type Container = {
  id: string;
  result: User | Post | null;
};

Generates:

type Container {
  id: String!
  result: ContainerResult
}
 
union ContainerResult = Post | User

Known Type References

When union members are exported types in the schema directory (knownTypeNames), they are preserved as references. Unknown inline object types are automatically generated:

import type { User } from "./User"; // known type
 
export type Activity = {
  actor: User | { id: string; type: string }; // User is referenced, anonymous object is generated
};

Generates:

type Activity {
  actor: ActivityActor!
}
 
union ActivityActor = ActivityActorAnonymous | User
 
type ActivityActorAnonymous {
  id: String!
  type: String!
}

Validation

Inline unions in output context must contain only object types. The following are not allowed:

  • Primitive types (string, number, boolean)
  • Enum types
  • Scalar types
// These will produce errors:
export type Invalid = {
  value: string | number;        // Error: primitives not allowed
  status: User | "active";       // Error: cannot mix object and enum
};

Union vs Enum

  • Use Union when each member is a distinct object type with different fields
  • Use Enum when you need a set of string constants
// Union: Different object types
export type SearchResult = Post | Comment | User;
 
// Enum: String constants
export type Status = "ACTIVE" | "INACTIVE" | "PENDING";

Union vs Interface

  • Use Union when types share no common fields
  • Use Interface when types share common fields that should be queryable
// Union: No common structure required
export type SearchResult = Post | Comment;
 
// Interface: Common fields enforced
export type Node = GqlInterface<{
  id: IDString;
}>;

See Interfaces for more details on interface types.

Inline Union Payloads

When a resolver return type is an inline union with object literals, gqlkit generates a GraphQL Union type. Each union member must have a __typename property with a string literal type:

export const updateUser = defineMutation<
  { input: UpdateUserInput },
  | { __typename: "UpdateUserSuccess"; user: User }
  | { __typename: "UpdateUserError"; message: string }
>(/* ... */);

Generates:

type Mutation {
  updateUser(input: UpdateUserInput!): UpdateUserPayload!
}
 
union UpdateUserPayload = UpdateUserError | UpdateUserSuccess
 
type UpdateUserSuccess {
  user: User!
}
 
type UpdateUserError {
  message: String!
}

__typename Requirement

For inline union payloads, the __typename property is required and must be a string literal type:

// ✅ OK: __typename with string literal type
type Result =
  | { __typename: "Success"; data: string }
  | { __typename: "Error"; message: string };
 
// ❌ Error: __typename missing
type Invalid =
  | { data: string }
  | { message: string };
 
// ❌ Error: __typename is not a string literal
type AlsoInvalid =
  | { __typename: string; data: string }
  | { __typename: string; message: string };

Automatic __resolveType Generation

For inline union payloads, gqlkit automatically generates a __resolveType function that returns the __typename property value. You don’t need to define it manually.

If you need custom type resolution logic, use defineResolveType.

Mixed Union Payloads

You can mix inline object literals with named types. Only inline objects require __typename:

import type { User } from "./user";
 
export const findEntity = defineQuery<
  { id: string },
  | User // named type - __typename determined by type
  | { __typename: "Guest"; sessionId: string } // inline type - __typename required
>(/* ... */);

Generates:

union FindEntityPayload = Guest | User
 
type Guest {
  sessionId: String!
}

See Queries & Mutations for more details on inline payload types.

Runtime Type Resolution

When GraphQL executes a query that returns a union type, it needs to determine the concrete type at runtime.

Automatic Resolution

If your union member types have __typename or $typeName fields with string literal values, gqlkit automatically generates the resolveType function:

export interface User {
  __typename: "User";
  id: string;
  name: string;
}
 
export interface Post {
  __typename: "Post";
  id: string;
  title: string;
}
 
export type SearchResult = User | Post;
// resolveType is automatically generated - no manual definition needed

Custom Discriminator Fields

When union member types use a field other than __typename or $typeName for discrimination (e.g., external library types like AI SDK’s UIMessagePart), you can configure custom discriminator fields via gqlkit.config.ts. gqlkit will automatically generate __resolveType based on the specified fields.

// gqlkit.config.ts
import { defineConfig } from "@gqlkit-ts/cli";
 
export default defineConfig({
  discriminatorFields: {
    ContentPart: "type",
  },
});
// src/gqlkit/schema/types.ts
export interface TextPart {
  type: "text";
  text: string;
}
 
export interface ImagePart {
  type: "image";
  url: string;
  alt: string;
}
 
export type ContentPart = TextPart | ImagePart;

Generates a switch-based __resolveType:

ContentPart: {
  __resolveType: (obj) => {
    switch (obj.type) {
      case "text": return "TextPart";
      case "image": return "ImagePart";
      default: return undefined;
    }
  },
},

Unlike __typename or $typeName, custom discriminator fields remain as regular GraphQL fields in the generated schema:

union ContentPart = ImagePart | TextPart
 
type TextPart {
  type: String!
  text: String!
}
 
type ImagePart {
  type: String!
  url: String!
  alt: String!
}

Multiple Discriminator Fields

When a single field is not enough to uniquely identify each member, you can specify multiple discriminator fields as an array. The first field must exist on all members with a string literal type. Secondary fields do not need to exist on every member.

// gqlkit.config.ts
import { defineConfig } from "@gqlkit-ts/cli";
 
export default defineConfig({
  discriminatorFields: {
    Content: ["type", "mediaType"],
  },
});
export type Content =
  | { type: "text"; mediaType: "plain"; body: string }
  | { type: "text"; mediaType: "html"; html: string }
  | { type: "image"; url: string; alt: string };

Generates nested switch statements and type names derived from the discriminator values:

union Content = ContentImage | ContentTextHtml | ContentTextPlain
 
type ContentTextPlain {
  type: String!
  mediaType: String!
  body: String!
}
 
type ContentTextHtml {
  type: String!
  mediaType: String!
  html: String!
}
 
type ContentImage {
  type: String!
  url: String!
  alt: String!
}

Validation Rules

  • The first discriminator field must exist on all union members and have a string literal type
  • Secondary fields do not need to exist on every member, but the combination of values must be unique across all members
  • If a defineResolveType is manually defined for the same union, the manual definition takes priority
  • If discriminatorFields is configured for a union that also has $typeName or __typename, the discriminatorFields configuration takes priority

See Configuration for the full config reference.

Manual Resolution

For types without __typename or $typeName, use defineResolveType:

import { defineResolveType } from "../gqlkit";
 
export const searchResultResolveType = defineResolveType<SearchResult>(
  (value) => {
    if ("name" in value) return "User";
    return "Post";
  }
);

See Abstract Type Resolution for complete documentation.

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