Module | Sequel::EmulateOffsetWithReverseAndCount |
In: |
lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb
|
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.
# File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 6 6: def empty? 7: if o = @opts[:offset] 8: unlimited.count <= o 9: else 10: super 11: end 12: end
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.
# File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 20 20: def select_sql 21: return super unless o = @opts[:offset] 22: 23: order = @opts[:order] || default_offset_order 24: if order.nil? || order.empty? 25: raise(Error, "#{db.database_type} requires an order be provided if using an offset") 26: end 27: 28: ds = unlimited 29: row_count = @opts[:offset_total_count] || ds.clone(:append_sql=>'').count 30: dsa1 = dataset_alias(1) 31: 32: if o.is_a?(Symbol) && @opts[:bind_vars] && (match = Sequel::Dataset::PreparedStatementMethods::PLACEHOLDER_RE.match(o.to_s)) 33: # Handle use of bound variable offsets. Unfortunately, prepared statement 34: # bound variable offsets cannot be handled, since the bound variable value 35: # isn't available until later. 36: s = match[1].to_sym 37: if prepared_arg?(s) 38: o = prepared_arg(s) 39: end 40: end 41: 42: reverse_offset = row_count - o 43: ds = if reverse_offset > 0 44: ds.limit(reverse_offset). 45: reverse_order(*order). 46: from_self(:alias=>dsa1). 47: limit(@opts[:limit]). 48: order(*order) 49: else 50: # Sequel doesn't allow a nonpositive limit. If the offset 51: # is greater than the number of rows, the empty result set 52: # shuld be returned, so use a condition that is always false. 53: ds.where(1=>0) 54: end 55: sql = @opts[:append_sql] || '' 56: subselect_sql_append(sql, ds) 57: sql 58: end