AR-Association-Practice

active-record-association-practice.md

Active Record Associations Practice

Associations let us access our data in diverse and flexible ways, simplifying our queries and allow some of our CRUD functionality.

When we are creating apps we start with a map of our initial tables and build them out one at a time.

After we create a table we add any necessary associations. To get some practice, we will start by drawing an ER Diagram, determining the proper associations/relationships between models and test them out in pry.

Note: These are often questions you may get in a technical interview when it comes to modeling a database for an app.

Questions to ask yourself when determining the relationships and validations/restrictions between objects?

• What data do I want to be able to access from these relationships?
• Does "a" have many of "b"? Does "b" have many of "a"?
• Can "a" exist without "b"? Can "b" exist without "a"?

Example 1: Pokemaster & Pokemon

• Pokemasters

  • have a name and gym listed
  • can have multiple pokemon

• Pokemons

  • have a name, poketype, and ability
  • can be wild or belong to a pokemaster

• Queries:

  • Query a Pokemaster's collection of pokemon
  • Query to find out who a pokemon's pokemaster is
  • Query to find all wild pokemon

Example 2a: Doctors & Patients

• Doctors

  • Have a first and last name, specialty and location
  • Have many patients

• Patients

  • Have a first and last name
  • Have many doctors

• Queries:

  • Query a doctor's patients through relationships
  • Query a patient's doctors through relationships

Example 2b: Adding Appointments

• Appointments

  • must have a date, time, and location
  • can optionally have a patient listed
  • has to have a specific doctor

• Queries:

  • Query a doctor's patients through relationships & appointments
  • Query to find out what patient is going to an appointment
  • Query to find what appointments are available
  • Query to find what doctors have appointments available

Practice Example: Students, Professor & Office Hours

• Professors

  • has a first and last name
  • has a field of study
  • has many students

• Students

  • has a first and last name
  • has a expected graduation year
  • has many professors

• Office Hours Appointment

  • have to have a date, time
  • have to have a professor
  • can optionally have a student attend

• Queries:

  • Query a Professor's students
  • Query a Student's professors
  • Query to find what professor is in charge of a office hour
  • Query to find out what student is attending an office hour
  • Query to find open slots (without a student)