Kata

You are about to write a QuerySet-like object known from Django. Obviously, it is not that easy to implement and test a real database on codewars, but this kata is more about class magic methods and general filtering concept, than it is about databases.

You have a Person model with the following fields:

• first_name (string),
• last_name (string),
• age (int)

The Person model defined a objects property, which returns your ResultSet class instance. That ResultSet represents data fetched from the "database" (in this example this is just a list of instances, but hey, that's all I can do).

You have to define the following methods:

• first(), returning the first entry from the database
• last(), returning the last entry from the database
• filter(), returning a filtered "list" (we'll get to that later) of results. If an argument is None it should not be filtered.
• get() returning the first element matched. It should raise a ValueError if more than one entry is found, and a TypeError if no entries are found in the database.
• exists() returning True if at least one matching record is found.

The filter() and get() methods should accept the following optional arguments:

• first_name
• last_name
• age

If any of the above is ommitted it should not be filtered.

Now, for the tricky part:

• filter() calls should be chainable. It should be possible to either use .filter(first_name='A', age=20) or .filter(first_name='A').filter(age=20).
• get(), first() and last() should return a single Person instance.
• .exists() should be possible to get called on any ResultSet instance, wheter it is filtered or not.
• It should be possible to iterate over a ResultSet to read all its results.
• It should be able to ask for a ResultsSet item like from a list, to get the n-th result.