• Use ActiveRecord's AREL library to build optimized queries

In programming, a good maxim is this:

Use the best tool for the job

For example, you don't want to use JavaScript to build a computer for flying to the Moon. JavaScript doesn't have very good decimal precision and, at distances as far as the Moon, getting a number off in the hundred-thousandths point after the decimal is the difference between landing on that celestial orb or taking a long trip through nothing, forever.

Databases are AMAZING at linking and summarizing data. Ruby is a nice general-purpose programming language. So when we need to get data from a database, we want to ask the DATABASE to do as much of that work as possible. That's what it's good at. That's what it likes to do. It has sacrificed some capabilities in order to do other capabilities extremely well.

If you use this code:

doctors = Doctor.all
first_six_drs = doctors[0..5]

You will get you six doctors by using RUBY to "section off" six doctors using Ruby's range method ([]). But under the covers we asked the database for all the doctors and then took six of them. Wouldn't it make more sense to ask the database to get us only six Doctors in the first place? That's what the following code does:

Doctor.limit(6).to_a

Functions like limit are provided by the "AREL" engine that's built into ActiveRecord. AREL stands for "A Relational Algebra." If that sounds like some complex, awesome Mathematics and set theory stuff, it is! Fortunately, we don't have to get advanced degrees in mathematics to benefit from this engine. AREL lets us query the database, via ActiveRecord in an object-oriented-looking way but that uses as much of the database's power as possible.

In this lab, we've provided the solution (commented out) to the tests. You should step through the tests and "fix" each method to make the test pass.

As you uncomment, be sure to evaluate the implementation we've provided you. Methods like order, where, includes are all part of the AREL engine. You should look up these methods in the AREL documentation, and see how they're working to filter the data retrieved from the database before the result "gets to Ruby-land."

While it's not necessary to memorize all the chainable methods AREL provides ActiveRecord, it's best to know some of the common methods you saw in this lab. If you are working in a Rails environment realizing that AREL can make your queries more efficient can literally speed up your applications 1000x!

• AREL Documentation
• ThoughtBot - Using Arel to Compose SQL Queries

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