Can we add different fields to instances of the same entity?

Question

I have a City entity with two fields (Title and Population), and I have created two instances of that entity.

Can I add different fields to these two instances?

Thank for you responses but I want different approach Let me put my question in this way

1) I have entity: City (with fields Population and Title)

2) of which I created two content (what should i call Chicago an New York ? contens of type City or anything else??):

Chicago and New York

3) now i know the entity City is fieldable i.e i can add custome fields to it, but those will be accesible to both ‘Chicago’ and ‘New York

4) What I want is ‘Chicago’ and ‘New York’ also be fieldable and have distinct field as well as common field

5) so, it should look like as Chicago : (Title - ‘Chicago’, Population - 1000, CField1 - ‘Hello’) New York: (Title - ‘Chicago’, Population - 1000, NField1 - ‘Hello’, NField2 - ‘Hello’)

Please tell me if above requirement is possible to implement

Answer 1

In the same way fields are added to content types, and not to node instances, fields are added to entity's bundles, not to entity instances.

If you don't want to use two different bundles for the same entity (which IMO is the better solution), you can implement hook_field_access(), and make some fields editable/visible basing on on the value of some entity properties, such as the title, or the entity ID.
For example, the following code would make a field visible, and editable, only when the title property of the entity is equal to "New York"; not even the user #1 would be able to see it. Practically, it is like the field is not present.

function mymodule_field_access($op, $field, $entity_type, $entity, $account) {
  if ($field['field_name'] == 'your field' && entity_type == 'your entity type' && $entity->title == "New York") {
    return FALSE;
  }

  return TRUE;
}

This example is very simplified. You should normally check the value of $op, just in case future versions of Drupal introduce new values for that parameter.

Answer 2

This single entity will use all of the fields you attach to it – it's still 1 instance (what we call a 'bundle') of an entity.

If you want to conditionally allow fields to be displayed on the entity edit form (or the display of the entity) use the Conditional Fields module: http://drupal.org/project/conditional_fields

If you really want to use different sets of fields, you need 2 different bundles, i.e. a "city" entity of bundle "foo" and a "city" entity of bundle "bar".