Useful Rails 5 Expressions

Select All Fields (columns)  in a Table

def index
   @orders = Order.all
end

Select Some Fields (columns)  in a Table

def index
   @orders = Order.select(:product, :price, :quantity)
end

More at http://blog.bigbinary.com/2016/05/24/rails-5-adds-active-record-ignored-columns.html

Display All Rows  in a Table

def list
   @orders = Order.all
end

The @orders = Order.all line in the list method tells Rails to search the orders table
and store each row it finds in the @orders instance object.

Display Target Row  in a Table
 
def show
   @orders = Order.find(params[:id])
end

The @orders = Order.find(param[:id]) line in the show method tells Rails to search 
the orders table and store the target row it finds in the @orders instance object.