How do you get the nid from an entity id?

Question

I've got a field collection attached to a node. Given the entity ID of a given instance of that collection, how do I obtain the nid to which it's associated?

When using entity_load, the nid for some reason is a protected property, so I can't access it. I must be missing something.

Update: using node_load on this entity ID returns false.

Answer 1

It's important to understand that if all you have is the id of any entity, then there is no way to load a specific entity with that id. Since entity_id's are not unique across entity-types, it is necessary to know both the entity type, and the entity id.

See Developers need an $entity->entity_type property for a discussion on changing this for D7, as well as info about how this will be likely be handled in D8.

On the other hand, as pointed out in a different answer, if you already know that the entity id is a node id, then you can call an entity specific load function like node_load($nid). Since most entities will have such a load function to support named arguments, such a "shortcut" function will be almost universally available.

Examples: node_load(), taxonomy_term_load(), commerce_product_load().

Answer 2

In this example Classroom is a content type (entity) with a field called field_students. field_students is a node reference field, and it references nodes (entities) of content type Student.

// Get all the Student items attached to a field. 
$studets = field_get_items('node', $classroom, 'field_students');

// Get the node id of the Student entity in the first field entry.
$student_nid = $studets[0]['nid'];

// Create a full object (node) representation of the student.
$student_obj = node_load($student_nid);