create.rb

# app/graph/mutations/post_mutations/create.rb
module PostMutations
  Create = GraphQL::Relay::Mutation.define do
    # Used to name derived types, eg `"CreatePostInput"`:
    name 'CreatePost'
    description 'Create post with a title and return a post'

    # Define input parameters
    # Accessible from `input` in the resolve function:
    input_field :title, !types.String

    # Define return parameters
    # The result has access to these fields,
    # resolve must return a hash with these keys
    return_field :post, PostType

    # Resolve block to create post and return hash of post
    # The resolve proc is where you alter the system state.
    resolve -> (_obj, inputs, ctx) {
      # Pundit authorization
      ctx[:pundit].authorize :post, :create?

      post = ctx[:current_user].posts.create!(title: inputs[:title])

      # The keys in this hash correspond to `return_field`s above:
      {
        post: post
      }
    }
  end
end