Working with DataObject
models
- Adding DataObject models to the schema
- An overview of how the DataObject model can influence the creation of types, queries, and mutations
- DataObject query plugins
- Learn about some of the useful goodies that come pre-packaged with DataObject queries
- DataObject operation permissions
- A look at how permissions work for DataObject queries and mutations
- Property mapping and dot syntax
- Learn how to customise field names, use dot syntax, and use aggregate functions
- Nested type definitions
- Define dependent types inline with a parent type
- DataObject inheritance
- Learn how inheritance is handled in DataObject model types
- Versioned content
- A guide on how DataObject models with the Versioned extension behave in GraphQL schemas
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.
A good way to determine whether you need an inline fragment is to ask "can this field appear on any other types in the query?" If the answer is yes, you want to use an interface, which is usually the parent class with the "Interface" suffix.
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
}
}
}
}
}
}
}
The above example shows a query for elements on all elemental pages - but for most situations you will probably only want to query the elements on one page at a time.
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).