Hybrid Structures
Admittedly, the terminology is getting a little over-the-top, but it is important to dedicate a separate category to two structures because they behave somewhat like simple structures and somewhat like Superstructures.
The two Hybrid Structures are the Association (single) and the
Association (multi) structures. These structures allow you to associate
one entry with an entry in another collection. Their usage is somewhat
analogous to belongs_to and has_many in Ruby on Rails and
INNER JOINs in database terminology.
Within their parent, they behave like a simple structure. They produce an editor widget on the editor screen just like any other simple structure. However, the associated entries can also be browsed on the associated collection.
For example, imagine a top level Collection called Categories, with
only one Text structure inside it called Name. Also, imagine a top
level Collection called Items with a Text structure called Name, and
an Association (single) structure associating the item with a category
from the Categories collection. This makes intuitive sense: the item
belongs to a category. So when browsing the Items collection, the
Association structure works like a simple structure. It renders a simple
editor widget on the create and edit pages, and displays the name of the
associated category in the list table view. However, when browsing the
Categories collection, the association will render like a Collection.
Next to each category in the list view will be a link to Show Items,
just as there would have been if the Categories collection had
contained a child collection called Items. Actually, this behavior
only happens when a value is provided for Reverse Association Name in
the configuration of the Association, but understanding this is key to
understanding the power of associations.
The difference between the Association (single) structure and the Assocation (multi) structure is pretty obvious: the Association (single) associates the entry with one other entry, while the Association (multi) associates the entry with zero or more entries.