Models in MVC
==============

In the previous chapters, you learned about :ref:`thymeleaf`, which display data and an
interface for a user, and :ref:`controllers` which determine what data to send the
the views. This data needs to come from some source and take some shape. Cue the models.

What is a Model?
-----------------

.. index:: model

A **model** represents the logic for accessing and storing the data used in an application. 
Properly constructed, they do not depend on any controllers or views, which
makes models easy to reuse without modification. 

Models are not the data itself, but rather the data logic. They dictate how we want to handle 
the data in an application-specific way. The data used in an application is often sourced from a database
or an external data service. Data is typically application-agnostic and it is the work of the models
we write to shape raw data into useful application information.

Consider a physical address book (a model). It stores names, addresses, phone
numbers, etc. on its pages. Anyone (a controller) can pick up the book,
retrieve the information, and then write some of the data on a separate piece
of paper (a view). The address book model does not depend on who picks it up and enters
their contacts. The book just provides organization and storage logic. On the flip side, the same
person can input the same contact data into a different book. So a model takes raw information and 
turns it into something useful for a particular application.

MVC: Putting it Together
------------------------

.. figure:: figures/mvcOverviewDetail.png
   :scale: 50%
   :alt: MVC Summary

   Each MVC node carries its own responsibilities.

Model
~~~~~
Shapes data to fit the needs of an application.

View
~~~~
Displays data to the user. Via events, the user interacts with the view and updates the program 
data. The view communicates with the controller but not the model.

Controller
~~~~~~~~~~
Directs the flow of information between the view and the
model. It does not store the data or determine how to display it for the
user. It passes information retrieved from the view to update the model. 
And it passes information retrieved from the model to update the view.

.. admonition:: Tip

   Need further review? Check out `MVC for Noobs <https://code.tutsplus.com/tutorials/mvc-for-noobs--net-10488>`__.

Model vs. Controller
--------------------

One tricky part of using the MVC structure is deciding what code belongs in the
model and what belongs in the controller. In general, any code that deals with
data transfer or responding to user actions belongs in the controller. Code that
involves retrieving data from a database or service or organizing that data belongs 
with the model. 

In our ``coding-events`` application thus far, we've done all data handling inside of 
controller classes. However, most data manipulation should occur in model classes.
So we need to make a distinction between these actions. For any manipulations that must occur
regardless of a user's actions, that code belongs in the model. For changes
that vary depending on a user's request (e.g. multiplying vs. adding a set of
numbers), that code belongs in the controller.

Model code defines the logic for processes that the user never needs to see.
These include:

#. The mechanics for storing data.
#. The mechanics for retrieving data.
#. The mechanics for organizing data.
#. Updating or manipulating the data independent of any controller or view
   actions.

Controller code handles *requests* made by the user. These include:

#. "Please retrieve this information from the model.",
#. "Please store this according to the rules of the model.",
#. "Please delete this item.",
#. "Please change this item.",
#. "Please display this.",
#. "Please modify this data in a novel way".

Remember, the controllers are the traffic cops, directing information from one place to another. 
A controller does not permanently store data. Instead, it either
sends the information to the model, which uses its own code to preserve the
data, or it sends the content to the view for display.

Check Your Understanding
------------------------

.. admonition:: Question

   If we use baking as an analogy for an MVC project, which of the
   following items best represents a *model*?

   a. The bread ingredients: flour, water, yeast, salt
   b. Mixing and shaping the ingredients together
   c. Baking the loaves into the final product
   d. Eating the bread

.. Answer: b, Mixing and shaping the ingredients together

.. admonition:: Question

   If we use a library as an analogy for an MVC project, which of the
   following items best represents a *model*?

   a. The books on the shelves
   b. The Dewey Decimal storage system
   c. The librarians
   d. The book readers

.. Answer: b, The Dewey Decimal storage system