05_test_driving_find_method.md

Test-driving a Repository class "find" method

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Learn to test-drive a "Repository" class method to SELECT a single record from the database.

Intro

You previously learned how to test-drive a Repository class method to retrieve all records from a table (we named this method all).

In this section, you will learn how to test-drive a method find that returns a single record using its id. It will perform a filtered SELECT query to retrieve that single row.

Here is the expected behaviour for this method:

repository = StudentRepository.new 

student = repository.find(1) # Performs a SELECT query and returns a single Student object.

student.id # 1
student.name # "David"

And its design:

Method	Job	Arguments	SQL query it performs	Returns
find	Get one student by ID	An id (number)	SELECT ...	A single Student

Micro Exercise

Write the SQL query to select the record with id 1 from the albums table.

SQL parameters

We need our Ruby method to receive the id to find as an argument, and build a SQL query that uses this value to find the right record.

We'll use what is called a SQL parameter. Here's an example of a "parameterised" query:

SELECT name, cohort_name FROM students WHERE id = $1;

The placeholder $1 is then dynamically replaced by our program when executing the query. Here's how it looks like once implemented with Ruby:

# file: lib/student_repository.rb

class StudentRepository

  def find(id)
    # The placeholder $1 is a "parameter" of the SQL query.
    # It needs to be matched to the corresponding element 
    # of the array given in second argument to exec_params.
    #
    # (If we needed more parameters, we would call them $2, $3...
    # and would need the same number of values in the params array).
    sql = 'SELECT name, cohort_name FROM students WHERE id = $1;'

    params = [id]

    result = DatabaseConnection.exec_params(sql, params)

    # (The code now needs to convert the result to a
    # Student object and return it)
  end
end

Demonstration

Here's a video demonstration.

Exercise

Work in the project directory music_library you've worked on before.

Use the Design recipe document used to test-drive the AlbumRepository#all method previously — following the steps 5 to 8 for the new AlbumRepository#find method.

1. Test-drive and implement the method find on the AlbumRepository class.
2. Modify the program in app.rb so it prints out to the terminal the data of the record with id 3, using the new method.

Example Video solution

Challenge

To work on this challenge, first:

• Setup a new project directory recipes_directory.
• Create a new database recipes_directory.
• Create a new test database recipes_directory_test for your tests.

Then:

1. Design and create the table for the following user stories.

   As a food lover,
   So I can stay organised and decide what to cook,
   I'd like to keep a list of all my recipes with their names.
   
   As a food lover,
   So I can stay organised and decide what to cook,
   I'd like to keep the average cooking time (in minutes) for each recipe.
   
   As a food lover,
   So I can stay organised and decide what to cook,
   I'd like to give a rating to each of the recipes (from 1 to 5).
   

2. Test-drive the Repository class for this new table. You should design, test-drive and implement two methods all and find.

3. Write code in the main file app.rb so it prints out the list of recipes.

Next Challenge

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