# virtual_rows.rdoc

Path: | doc/virtual_rows.rdoc
Last Update: | Tue Jul 21 15:24:40 -0600 2009

# Virtual Row Blocks

Dataset methods filter, order, and select all take blocks that yield instances of Sequel::SQL::VirtualRow. These are referred to as virtual row blocks. Many other dataset methods pass the blocks they are given into one of those three methods, so there are actually many Sequel methods that take virtual row blocks.

VirtualRow is a class that returns SQL::Indentifiers, SQL::QualifiedIdentifiers, or SQL::Functions depending on how it is called. This is best shown by example:

      ds = DB[:items]
      ds.filter{|o| o.column > 1}        # column > 1
      ds.filter{|o| o.table__column > 1} # table.column > 1
      ds.filter{|o| o.function(1) > 1}   # function(1) > 1

Basically, the rules are:

- If there are arguments, an SQL::Function is returned with the name of the method used and the arguments given.
- If there are no arguments and the method contains a double underscore, split on the double underscore and return an SQL::QualifiedIdentifier with the table and column.
- Otherwise, create an SQL::Identifier with the name of the method.

One of the consequences of these rules is that you cannot create an SQL::Function that takes no arguments using a VirtualRow instance.

In Sequel 2.12, the following is deprecated by default, as virtual row blocks are required to accept an argument:

      ds.filter{:column > 1}                     # column > 1
      ds.filter{:table__column > 1}              # table.column > 1
      ds.filter{:function.sql_function(1) > 1}   # function(1) > 1

This is to keep backwards compatibility while notifying people to change their code. In Sequel 3.0, you will be able to do:

      ds.filter{column > 1}        # column > 1
      ds.filter{table__column > 1} # table.column > 1
      ds.filter{function(1) > 1}   # function(1) > 1

Sequel 3.0 will change the virtual row block support to use instance_eval if the block doesn‘t take an argument. This breaks backward compatibility because instance methods called inside the block will now have a virtual row instance as a receiver instead of the receiver where the block was defined. You can still use local variables in the enclosing scope inside the block, as they take on their usual meaning.

If you find yourself needing to call instance methods for the current receiver inside a virtual row block, you should change the block to accept an argument, and use that argument whenever you need the virtual row support.

You can get the Sequel 3.0 behavior in Sequel 2.12 using the following method:

      Sequel.virtual_row_instance_eval = true

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