On Wed, Jun 27, 2012 at 8:57 AM, David Wagner <cptnwi...@gmail.com> wrote: > With that said, I'm having trouble wrapping my brain around the ManyToMany > and Many-To-One and OneToOne concepts. I'm trying to create a multi-select > option for a user profile in which a user can have multiple attributes of a > certain kind. Right now I'm just working on the model. This is what I have, > is it right?
these terms are used mainly in the relational database design. as you might know, you can't have real arrays or structures on traditional SQL databases, so you use those three types of 'links'. the simplest is the 'many to one', you have one 'parent' table and a 'child' one with a field (called the "foreign key" field) where each record keeps a copy of the primary key of a 'parent' record. a common example is to have 'group' and 'member' tables. each 'member' record has a field (sometimes called 'group_id') that tells which to which group it belongs. with a non-unique index on that field, it's easy to pick all members of each group. a slight variation is the 'one to one'. here, the index on the foreign key field is a 'unique' index, that forbids two records with the same value, effectively making it impossible for a 'parent' record to have more than one 'child'. sometimes instead of defining a specific foreign key field, you assign the same primary key value to related records. the last one is the 'many to many'. for example, you can have users that belong to more than one group; and of course each group has many users. for this, you have to define a 'link table' where each record has two foreign key fields, one for each linked table. sometimes the primary key of the link table is just the composite index of these two foreign keys. also there can be a second composite index with the same foreign keys in the opposite order. Also you can choose to add some extra fields to the link table to express some properties of the link itself. In Django, you handle this just by declaring fields of the appropriate type; Django takes care of declaring the indexes, link table if needed, and a 'reverse relationship'. that last feature is the one that makes the 'members' pseudo array to magically appear in 'group' objects. hope this helps with the theory and makes the Django docs easier to read. -- Javier -- You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-users@googlegroups.com. To unsubscribe from this group, send email to django-users+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-users?hl=en.
