module Sequel::EmulateOffsetWithReverseAndCount

  1. lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb
Parent: Sequel

Methods

Public Instance

  1. empty?
  2. select_sql

Public Instance methods

empty? ()

Make empty? work with an offset with an order. By default it would break since the order would be based on a column that empty does not select.

[show source] 
# File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 6
def empty?
  if o = @opts[:offset]
    unlimited.count <= o
  else
    super
  end
end
select_sql ()

Emulate OFFSET support using reverse order in a subselect, requiring a count of the number of rows.

If offset is used, an order must be provided, since it needs to be reversed in the subselect. Note that the order needs to be unambiguous to work correctly, and you must select all columns that you are ordering on.

[show source] 
# File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 20
def select_sql
  return super unless o = @opts[:offset]

  order = @opts[:order] || default_offset_order
  if order.nil? || order.empty?
    raise(Error, "#{db.database_type} requires an order be provided if using an offset")
  end

  ds = unlimited
  row_count = @opts[:offset_total_count] || ds.clone(:append_sql=>'').count
  dsa1 = dataset_alias(1)

  if o.is_a?(Symbol) && @opts[:bind_vars] && (match = Sequel::Dataset::PreparedStatementMethods::PLACEHOLDER_RE.match(o.to_s))
    # Handle use of bound variable offsets.  Unfortunately, prepared statement
    # bound variable offsets cannot be handled, since the bound variable value
    # isn't available until later.
    s = match[1].to_sym
    if prepared_arg?(s)
      o = prepared_arg(s)
    end
  end

  reverse_offset = row_count - o
  ds = if reverse_offset > 0
    ds.limit(reverse_offset).
      reverse_order(*order).
      from_self(:alias=>dsa1).
      limit(@opts[:limit]).
      order(*order)
  else
    # Sequel doesn't allow a nonpositive limit.  If the offset
    # is greater than the number of rows, the empty result set
    # shuld be returned, so use a condition that is always false.
    ds.where(1=>0)
  end
  sql = @opts[:append_sql] || ''
  subselect_sql_append(sql, ds)
  sql
end