Craft GraphQL APIs in Elixir with Absinthe/

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Subscriptions Triggers

Subscriptions Triggers

Learn how to use GraphQL to trigger subscriptions.

We'll cover the following...

Introduction to trigger subscriptions

Since we have already built our context and schema, we can jump directly to building the relevant mutation fields in the GraphQL schema and filling out each resolver.

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mutation do
  field :ready_order, :order_result do
    arg :id, non_null(:id)
    resolve &Resolvers.Ordering.ready_order/3
  end
  field :complete_order, :order_result do
      arg :id, non_null(:id)
      resolve &Resolvers.Ordering.complete_order/3
  end
# «Other fields»
end

Our :ready_order and :complete_order fields use new resolver functions from PlateSlateWeb.Resolvers.Ordering. Let’s add those:

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def ready_order(_, %{id: id}, _) do
  order = Ordering.get_order!(id)
  with {:ok, order} <- Ordering.update_order(order, %{state: "ready"}) do
    {:ok, %{order: order}}
  else
    {:error, changeset} ->
        {:ok, %{errors: transform_errors(changeset)}}
    end 
  end
def complete_order(_, %{id: id}, _) do 
  order = Ordering.get_order!(id)
  
  with {:ok, order} <- Ordering.update_order(order, %{state: "complete"}) do 
    {:ok, %{order: order}}
else
    {:error, changeset} ->
      {:ok, %{errors: transform_errors(changeset)}}
    end
 end

Subscribing to these events is slightly different from before because now we’re trying to handle events for specific orders based on ID. When the client is notified about new orders by a new_order subscription, we want to allow them to subscribe to future updates for each subscription.

We want to support a GraphQL document that looks like this:

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subscription {
  updateOrder(id: "13") {
    customerNumber
     state 
  }
}

Notably, we want to use this one subscription field to get updates triggered by both the :ready_order and :complete_order mutation fields. While it’s important to represent the mutations as different fields, we often just need a single subscription that lets us get all the state changes for ...