Post Object fields are added to the WPGraphQL Schema as a field with a Union of Possible Types the field is configured to allow.
If the field is configured to allow multiple selections, it will be added to the Schema as a List Of the Union Type.
Since Post Object fields can be configured to be limited to certain Post Types, the Union will represent those Types.
For example, if the Post Object field is configured to allow Posts of the post and page types to be selected:
Then the Union type for the field will allow Post and Page types to be returned, as seen in the Schema via GraphiQL:
Here, we have a Post Object field named post_object on the Post Edit screen within the "ACF Docs" Field Group, configured with the Post "Hello World!".
As a GraphQL consumer, we don't know in advance if the value is going to be a Page or a Post.
So we can specify, via GraphQL fragment, what fields we want if the object is a Post, or if it is a Page.
This field can be Queried in GraphQL like so:
{
post(id: "acf-example-test", idType: URI) {
acfDocs {
postObject {
__typename
... on Post {
id
title
date
}
... on Page {
id
title
}
}
}
}
}and the results of the query would be:
{
"data": {
"post": {
"acfDocs": {
"postObject": {
"__typename": "Post",
"id": "cG9zdDox",
"title": "Hello world!",
"date": "2020-02-20T23:12:21"
}
}
}
}
}If the input of the field was saved as a Page, instead of a Post, like so:
Then the same query above, would return the following results:
{
"data": {
"post": {
"acfDocs": {
"postObject": {
"__typename": "Page",
"id": "cGFnZToy",
"title": "Sample Page"
}
}
}
}
}Now, if the field were configured to allow multiple values, the field would be added to the Schema as a listOf, returning an Array of the Union.
If the field were set with a value of one Page, and one Post, like so:
Then the results of the same query as above would be:
{
"data": {
"post": {
"acfDocs": {
"postObject": [
{
"__typename": "Page",
"id": "cGFnZToy",
"title": "Sample Page"
},
{
"__typename": "Post",
"id": "cG9zdDox",
"title": "Hello world!",
"date": "2020-02-20T23:12:21"
}
]
}
}
}
}