Gain insights into property-based testing with PropEr in Elixir. Learn foundational principles, custom data generators, and advanced concepts for effective real-world application testing.

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Property-based testing relies on specifying some property of code, unlike unit tests, which specify the expected output in response to some inputs. In this course, we'll cover the concepts we need to feel confident about using even the most advanced features of PropEr with Elixir.

We'll start with the basic and foundational principles of property-based testing, see what the framework offers us to get started, make our way through thinking in properties, write our own custom data generators, and then learn more advanced concepts. We'll also see how you can use property-based testing in a realistic project.

By the time we finish this course, we should feel comfortable testing real-world applications using properties.

Property-Based Testing with PropEr in Elixir

## The application

The application just starts a standard supervisor without children. This is how it's done.

```elixir
defmodule Bookstore.App do
  use Application

  def start(_type, _args) do
    Bookstore.Sup.start_link()
  end

  def stop(_state) do
    :ok
  end
end
```

And now for the supervisor. Let's look at how that works. 

## The supervisor

This is what our supervisor would look like. 

```elixir
defmodule Bookstore.Sup do
  use Supervisor

  def start_link() do
    Supervisor.start_link(__MODULE__, [], name: __MODULE__)
  end

  def init([]) do
    Bookstore.DB.load_queries()
    Supervisor.init([], strategy: :one_for_one)
  end
end
```

Note that `Bookstore.DB.load_queries()` gets called. This function is not yet defined. Its role, though, will be to find all the SQL queries within a file we’ll define in `priv/queries.sql`, which are labeled by name in the comments.

## Queries

# The application

The application just starts a standard supervisor without children. This is how it's done.

```elixir
defmodule Bookstore.App do
  use Application

  def start(_type, _args) do
    Bookstore.Sup.start_link()
  end

  def stop(_state) do
    :ok
  end
end
```

And now for the supervisor. Let's look at how that works. 

# The supervisor

This is what our supervisor would look like. 

```elixir
defmodule Bookstore.Sup do
  use Supervisor

  def start_link() do
    Supervisor.start_link(__MODULE__, [], name: __MODULE__)
  end

  def init([]) do
    Bookstore.DB.load_queries()
    Supervisor.init([], strategy: :one_for_one)
  end
end
```

Note that `Bookstore.DB.load_queries()` gets called. This function is not yet defined. Its role, though, will be to find all the SQL queries within a file we’ll define in `priv/queries.sql`, which are labeled by name in the comments.

# Queries

Take a look at the basics of the application and the SQL queries that the application will be using.

The Application: Getting Started

Foundations of Property-Based Testing

Writing Properties

Thinking in Properties

Custom Generators

Responsible Testing

Properties-Driven Development

Shrinking

Stateful Properties

Case Study: Bookstore

State Machine Properties

Conclusion

The Application: Getting Started

The application