Array Chicanery

Ruby's Sweet Sugar

So of course it is Sunday so I am working on my Bloc homework as Sunday is a day that ends in 'y'. The SET program as it stands is full of not just exposure to the theory and building blocks of computer science but also to the wonders of Ruby.

I saw this little nugget of gold in an exercise and had no idea what it did.

connection.execute <<-SQL
    UPDATE #{table}
    SET #{updates_array * ","}
    WHERE id = #{id};
SQL

Now the <<-SQL ... SQL bit is just raw SQL. Everything between the two SQLs is passed along as SQL and is not interpreted as Ruby. You could just as easily write the code below for identical results.

sql = "UPDATE #{table} SET #{updates_array * ","} WHERE id = #{id};"
connection.execute(sql)

Two other things to note here are the string interpolation and the array multiplication.

The string interpolation should be familiar from working with Rails where we do this sort of thing all the time in our Views. #{some_variable} pulls code from the Ruby side of the house into a string in this example, or into HTML in Rails.

What was completely new to me was the #{updates_array * ","} - what the heck is going on here? In the example, updates_array is an array of strings, with each string equivalent to some_key=some_value so as to perform a SQL UPDATE command. So what gives with the multiplication?

Join or Die

Well, it turns out that the multiplication and "," are the equivalent of join! We could write this as updates_array.join(",") and also get an identical result. Using this syntax allows us to convert the array into a single set of comma-separated strings.