Now that we have some understanding of how to describe a system, let's think a little bit more about the relationships between entities. For those of you who have not dealt with modeling a database before this will help you understand what fields you will need in our database.
As an example let's consider a simplified baseball league. When we were describing systems in the previous task we could say that the following elements exist: leagues, teams, players, and games. What are the entity relationships between these? Let's start from the bottom and work our way up.
Player- Can only play for one team at a time. Example: Jose Reyes plays for the NY Mets
Team - Belongs to only 1 league. Has multiple players. Example: The NY Mets belong to the National League. They have a roster of n players on their team.
Leagues - Has multiple teams. Example: The NY Mets, Philidelphia Phillies, Washington Nationals belong to the National League
Games - Contains any 2 teams from either league. Examle: The NY Mets vs Washington Nationals
So what does this mean for building web applications?
-
Player object would have a foreign key to a Team object
-
Team object would have a foreign key to a League object
-
Game object would have two fields that are foreign keys to a Team object
-
Team object could have a Many to Many field with players
In django (and databases in general), a relationship between models is called a foreign key. These are really important for being able to get the data we want back from our database.
Your assignment:
-
Choose a system (maybe reuse the one you did for Task 2)
-
Write down or diagram the entity relationships between the different elements
-
Post them
For those that are confused on this topic please take a look at this excellent example. It goes into much greater detail than I just did. http://uswaretech.com/blog/2010/01/django-models-tutorial/
This can be kind of a tricky subject, but once you get it the next step in building our web application will be easy!
