Cannot create a new entity (created with ECK) through API using REST module - drupal

Question

Here is my situation: I am using the ECK module with Drupal 8 to create entities and bundles, and the new REST core module to create API features.
I have installed the REST_UI module, and I enabled the route for the entity I am interested in.
Here's my issue: I created an entity type and a bundle with ECK, and I can then create a new entity when I am calling the /entity/entity_type_name endpoint with a POST request, giving the following parameter as json:
{
"type":[{"target_id":"bundle_name"}],
"field_test_text":[{"value":"test"}]
}
However, this is only working when I have only one entity type in my list of entities; Let's say for example I decide to create a new entity type, then run the same request, I got the following error message:
Drupal\Core\Entity\Exception\AmbiguousEntityClassException: Multiple entity types found for Drupal\eck\Entity\EckEntity
I understand that apparently, now that I have multiple entity types, the Entity API is not able to understand what should be the type of the entity it has to create (which I find pretty weird, considering that I am providing it in the URL under this form /entity/entity_type_name and that there are different routes available for the different types of entities that I have).
I guess I need to pass an extra parameter in my json for Drupal to understand what kind of entity it should create, but what would be this parameter ? I've been trying to look online and in the documentation, but I cannot figure out how to do this.

Answer 1

I had the same problem, and here is how I resolved it:
Enable the HAL module.
Enable hal_json under Accepted request formats in /admin/config/services/rest for
that particular resource.
Then, in your POST request, use headers:
Content-Type: application/hal+json
X-CSRF-Token: [AUTH SESSION TOKEN]
And the body of the request being:
{
"_links": {
"type": {
"href": "http://localhost:8080/rest/type/[ENTITY_TYPE]/[ENTITY_BUNDLE]"
}
},
"title":[
{"value": "This is a new entity title"}
],
"field_example":[
{"value": "This is an example of a custom text field value."}
]
}
Drupal is reading the entity type and bundle from the _links.type.href string.
For example, if your entity type was automobile and your bundle was car, your URL would be "http://localhost:8080/rest/type/automobile/car"