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Mutations

Mutations are the write side of your GraphQL API. They follow the same resolver pattern as queries but are conventionally used for operations that modify data.

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Defining Mutations

Create a mutation type the same way you create a query type:
Mutation Type
let mutationType = PrismGraphQLObjectType(
    name: "Mutation",
    fields: [
        "createUser": PrismGraphQLField(
            name: "createUser",
            type: .object("User"),
            args: [
                PrismGraphQLArgument(name: "name", type: .nonNull(.string)),
                PrismGraphQLArgument(name: "email", type: .nonNull(.string))
            ],
            resolver: { info in
                let name = info.arguments["name"] as? String ?? ""
                let email = info.arguments["email"] as? String ?? ""
                
                let db = info.context as? PrismDatabase
                try await db?.execute(
                    "INSERT INTO users (name, email) VALUES (?, ?)",
                    parameters: [.text(name), .text(email)]
                )
                let id = await db?.lastInsertID ?? 0
                return ["id": "\(id)", "name": name, "email": email]
            }
        )
    ]
)

let schema = PrismGraphQLSchema(query: queryType, mutation: mutationType)
Client mutation: 
mutation {
  createUser(name: "Alice", email: "alice@example.com") {
    id
    name
  }
}

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CRUD Example

Here’s a complete set of mutations for a Post resource:
Post CRUD Mutations
let mutations = PrismGraphQLObjectType(
    name: "Mutation",
    fields: [
        // CREATE
        "createPost": PrismGraphQLField(
            name: "createPost",
            type: .object("Post"),
            args: [
                PrismGraphQLArgument(name: "title", type: .nonNull(.string)),
                PrismGraphQLArgument(name: "body", type: .string),
                PrismGraphQLArgument(name: "authorId", type: .nonNull(.id))
            ],
            resolver: { info in
                let title = info.arguments["title"] as? String ?? ""
                let body = info.arguments["body"] as? String ?? ""
                let authorId = info.arguments["authorId"] as? String ?? ""
                
                let db = info.context as? PrismDatabase
                try await db?.execute(
                    "INSERT INTO posts (title, body, author_id) VALUES (?, ?, ?)",
                    parameters: [.text(title), .text(body), .text(authorId)]
                )
                let id = await db?.lastInsertID ?? 0
                return ["id": "\(id)", "title": title, "body": body, "authorId": authorId]
            }
        ),
        
        // UPDATE
        "updatePost": PrismGraphQLField(
            name: "updatePost",
            type: .object("Post"),
            args: [
                PrismGraphQLArgument(name: "id", type: .nonNull(.id)),
                PrismGraphQLArgument(name: "title", type: .string),
                PrismGraphQLArgument(name: "body", type: .string)
            ],
            resolver: { info in
                let id = info.arguments["id"] as? String ?? ""
                let title = info.arguments["title"] as? String
                let body = info.arguments["body"] as? String
                
                let db = info.context as? PrismDatabase
                if let title {
                    try await db?.execute(
                        "UPDATE posts SET title = ? WHERE id = ?",
                        parameters: [.text(title), .text(id)]
                    )
                }
                if let body {
                    try await db?.execute(
                        "UPDATE posts SET body = ? WHERE id = ?",
                        parameters: [.text(body), .text(id)]
                    )
                }
                let row = try await db?.queryFirst(
                    "SELECT * FROM posts WHERE id = ?",
                    parameters: [.text(id)]
                )
                return row.map { ["id": $0["id"], "title": $0["title"], "body": $0["body"]] }
            }
        ),
        
        // DELETE
        "deletePost": PrismGraphQLField(
            name: "deletePost",
            type: .boolean,
            args: [
                PrismGraphQLArgument(name: "id", type: .nonNull(.id))
            ],
            resolver: { info in
                let id = info.arguments["id"] as? String ?? ""
                let db = info.context as? PrismDatabase
                let affected = try await db?.execute(
                    "DELETE FROM posts WHERE id = ?",
                    parameters: [.text(id)]
                )
                return (affected ?? 0) > 0
            }
        )
    ]
)

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Input Validation

Validate inputs before persisting data:
Validation in Resolvers
"createUser": PrismGraphQLField(
    name: "createUser",
    type: .object("User"),
    args: [
        PrismGraphQLArgument(name: "name", type: .nonNull(.string)),
        PrismGraphQLArgument(name: "email", type: .nonNull(.string)),
        PrismGraphQLArgument(name: "age", type: .int)
    ],
    resolver: { info in
        let name = info.arguments["name"] as? String ?? ""
        let email = info.arguments["email"] as? String ?? ""
        let age = info.arguments["age"] as? Int
        
        // Validate
        guard name.count >= 2 else {
            throw PrismAppError.badRequest("Name must be at least 2 characters")
        }
        guard email.contains("@") else {
            throw PrismAppError.badRequest("Invalid email address")
        }
        if let age, age < 0 || age > 150 {
            throw PrismAppError.badRequest("Age must be between 0 and 150")
        }
        
        // Persist...
        return ["id": "1", "name": name, "email": email]
    }
)
Always validate inputs in mutations. GraphQL type checking only verifies that arguments are the right type — it doesn’t enforce business rules like “email must be unique” or “name must be at least 2 characters.”

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Error Handling

Mutations that throw errors return structured error responses:
Error Responses
"transfer": PrismGraphQLField(
    name: "transfer",
    type: .boolean,
    args: [
        PrismGraphQLArgument(name: "from", type: .nonNull(.id)),
        PrismGraphQLArgument(name: "to", type: .nonNull(.id)),
        PrismGraphQLArgument(name: "amount", type: .nonNull(.float))
    ],
    resolver: { info in
        let amount = info.arguments["amount"] as? Double ?? 0
        
        guard amount > 0 else {
            throw PrismAppError.badRequest("Amount must be positive")
        }
        guard amount <= 10000 else {
            throw PrismAppError.badRequest("Amount exceeds transfer limit", code: "TRANSFER_LIMIT")
        }
        
        // Process transfer...
        return true
    }
)
The client receives: 
{
  "data": { "transfer": null },
  "errors": [{ "message": "Amount exceeds transfer limit", "path": ["transfer"] }]
}

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What’s Next

Playground

Test your mutations interactively with the built-in playground

Schema

Refine your schema with more types and relationships