Working with DataObject models#

DataObject inheritance#

The inheritance pattern used in the ORM is a tricky thing to navigate in a GraphQL API, mostly owing to the fact that there is no concept of inheritance in GraphQL types. The main tools we have at our disposal are interfaces and unions to deal with this type of architecture, and we leverage both of them when working with DataObjects.

Key concept: querying types that have descendants#

When you query a type that has descendant classes, you are by definition getting a polymorphic return. There is no guarantee that each result will be of one specific type. Take this example:

query {
  readPages {
    nodes {
      title
      content
    }
  }
}

This is fine when the two fields are common to across the entire inheritance chain, but what happens when we need the date field on BlogPage?

query {
  readPages {
    nodes {
      title
      content
      date # fails!
    }
  }
}

To solve this problem, the GraphQL module will automatically change these types of queries to return interfaces.

query {
  readPages {
    nodes { # <--- [PageInterface]
      title
      content
    }
  }
}

Now, in order to query fields that are specific to BlogPage, we need to use an inline fragment to select them.

In the below example, we are querying title and content on all page types, but we only query heroImage on HomePage objects, and we query date and author only for BlogPage objects.

query {
  readPages {
    nodes {
      title # Common field
      content # Common field
      ... on HomePage {
        heroImage {
          url
        }
      }
      ... on BlogPage {
        date
        author {
          firstName
        }
      }
    }
  }
}

So the fields that are common to every possible type in the result set can be directly selected (with no ...on syntax), because they're part of the common interface. They're guaranteed to exist on every type. But for fields that only appear on some types, we need to be explicit.

Now let's take this a step further. What if there's another class in between? Imagine this ancestry:

Page
  -> EventPage extends Page
     -> ConferencePage extends EventPage
     -> WebinarPage extends EventPage

We can use the intermediary interface EventPageInterface to consolidate fields that are unique to ConferencePage and WebinarPage.

query {
  readPages {
    nodes {
      title # Common to all types
      content # Common to all types
      ... on EventPageInterface {
        # Common fields for WebinarPage, ConferencePage, EventPage
        numberOfTickets
        featuredSpeaker {
          firstName
          email
        }
      }
      ... on WebinarPage {
        zoomLink
      }
      ... on ConferencePage {
        venueSize
      }
      ... on BlogPage {
        date
        author {
          firstName
        }
      }
    }
  }
}

You can think of interfaces in this context as abstractions of parent classes - and the best part is they're generated automatically. We don't need to manually define or implement the interfaces.

Inheritance: a deep dive#

There are several ways inheritance is handled at build time:

• Implicit field / type exposure
• Interface generation
• Assignment of generated interfaces to types
• Assignment of generated interfaces to queries

We'll look at each of these in detail.

Inherited fields / implicit exposures#

Here are the rules for how inheritance affects types and fields:

• Exposing a type implicitly exposes all of its ancestors.
• Ancestors receive any fields exposed by their descendants, if applicable.
• Exposing a type applies all of its fields to descendants only if they are explicitly exposed also.

All of this is serviced by: InheritanceBuilder

Example#

App\PageType\BlogPage:
  fields:
    title: true
    content: true
    date: true

App\PageType\GalleryPage:
  fields:
    images: true
    urlSegment: true

This results in those two types being exposed with the fields as shown, but also results in a Page type:

type Page {
  id: ID! # always exposed
  title: String
  content: String
  urlSegment: String
}

Interface generation#

Any type that's part of an inheritance chain will generate interfaces. Each applicable ancestral interface is added to the type. Like the type inheritance pattern shown above, interfaces duplicate fields from their ancestors as well.

Additionally, a base interface is provided for all types containing common fields across the entire DataObject schema.

All of this is serviced by: InterfaceBuilder

Example#

Page
  -> BlogPage extends Page
  -> EventsPage extends Page
     -> ConferencePage extends EventsPage
     -> WebinarPage extends EventsPage

This will create the following interfaces (assuming the fields below are exposed):

interface PageInterface {
  id: ID!
  title: String
  content: String
}

interface BlogPageInterface {
  id: ID!
  title: String
  content: String
  date: String
}

interface EventsPageInterface {
  id: ID!
  title: String
  content: String
  numberOfTickets: Int
}

interface ConferencePageInterface {
  id: ID!
  title: String
  content: String
  numberOfTickets: Int
  venueSize: Int
  venurAddress: String
}

interface WebinarPageInterface {
  id: ID!
  title: String
  content: String
  numberOfTickets: Int
  zoomLink: String
}

Interface assignment to types#

The generated interfaces then get applied to the appropriate types, like so:

type Page implements PageInterface {}
type BlogPage implements BlogPageInterface & PageInterface {}
type EventsPage implements EventsPageInterface & PageInterface {}
type ConferencePage implements ConferencePageInterface & EventsPageInterface & PageInterface {}
type WebinarPage implements WebinarPageInterface & EventsPageInterface & PageInterface {}

Lastly, for good measure, we create a DataObjectInterface that applies to everything.

interface DataObjectInterface {
  id: ID!
  # Any other fields you've explicitly exposed in config.modelConfig.DataObject.base_fields
}

type Page implements PageInterface & DataObjectInterface {}

Interface assignment to queries#

Queries, both at the root and nested as fields on types, will have their types updated if they refer to a type that has had any generated interfaces added to it.

type Query {
  readPages: [Page]
}

type BlogPage {
  download: File
}

Becomes:

type Query {
  readPages: [PageInterface]
}

type BlogPage {
  download: FileInterface
}

All of this is serviced by: InterfaceBuilder

Elemental#

This section refers to types added via dnadesign/silverstripe-elemental.

Almost by definition, content blocks are always abstractions. You're never going to query for a BaseElement type specifically. You're always asking for an assortment of its descendants, which adds a lot of polymorphism to the query.

query {
  readElementalPages {
    nodes {
      elementalArea {
        elements {
          nodes {
            title
            id
            ... on ContentBlock {
              html
            }
            ... on CTABlock {
              link
              linkText
            }
          }
        }
      }
    }
  }
}

Optional: use unions instead of interfaces#

You can opt out of using interfaces as your return types for queries and instead use a union of all the concrete types. This comes at a cost of potentially breaking your API unexpectedly (described below), so it is not enabled by default. There is no substantive advantage to using unions over interfaces for your query return types. It would typically only be done for conceptual purposes.

To use unions, turn on the useUnionQueries setting.

# app/_graphql/config.yml
modelConfig:
  DataObject:
    plugins:
      inheritance:
        useUnionQueries: true

This means that models that have descendants will create unions that include themselves and all of their descendants. For queries that return those models, a union is put in its place.

Serviced by: InheritanceUnionBuilder

Example#

type Page implements PageInterface {}
type BlogPage implements BlogPageInterface & PageInterface {}
type EventsPage implements EventsPageInterface & PageInterface {}
type ConferencePage implements ConferencePageInterface & EventsPageInterface & PageInterface {}
type WebinarPage implements WebinarPageInterface & EventsPageInterface & PageInterface {}

Creates the following unions:

union PageInheritanceUnion = Page | BlogPage | EventsPage | ConferencePage | WebinarPage
union EventsPageInheritanceUnion = EventsPage | ConferencePage | WebinarPage

"Leaf" models like BlogPage, ConferencePage, and WebinarPage that have no exposed descendants will not create unions, as they are functionally useless.

This means that queries for readPages and readEventsPages will now return unions.

query {
  readPages {
    nodes {
      ... on PageInterface {
        id # in theory, this common field could be done on DataObjectInterface, but that gets a bit verbose
        title
        content
      }
      ... on EventsPageInterface {
        numberOfTickets
      }
      ... on BlogPage {
        date
      }
      ... on WebinarPage {
        zoomLink
      }
    }
  }
}

Lookout for the footgun#

Because unions are force substituted for your queries when a model has exposed descendants, it is possible that adding a subclass to a model will break your queries without much warning to you.

For instance:

namespace App\Model;

class Product extends DataObject
{
    private static $db = ['Price' => 'Int'];
}

We might query this with:

query {
  readProducts {
    nodes {
      price
    }
  }
}

But if we create a subclass for product and expose it to GraphQL:

namespace App\Model;

class DigitalProduct extends Product
{
    private static $db = ['DownloadURL' => 'Varchar'];
}

Now our query breaks:

query {
  readProducts {
    nodes {
      price # Error: Field "price" not found on ProductInheritanceUnion
    }
  }
}

We need to revise it:

query {
  readProducts {
    nodes {
      ... on ProductInterface {
        price
      }
      ... on DigitalProduct {
        downloadUrl
      }
    }
  }
}

If we use interfaces, this won't break because the price field will be on ProductInterface which makes it directly queryable (without requiring the inline fragment).

Further reading#