Empty Filters

Historically, Sequel’s behavior for the following has violated many people’s principle of least surprise:

ds.filter     # raises Sequel::Error
ds.filter({}) # raises Sequel::Error
ds.filter([]) # raises Sequel::Error
ds.filter('') # invalid SQL: WHERE ()

Now who in their right mind would do that? Well, using the direct syntax above, probably nobody, but consider the following cases:

attrs = {}
attrs[:year] = year if year
attrs[:state] = state if state 
# same as ds.filter({}) if
# year and state are both nil.

attrs = []
attrs.push([:year, 40..60]) if middle_aged?
attrs.push([:year, 55..75]) if nearing_retirement?
# same as ds.filter([]) if
# middle_aged? and nearing_retirement? are both
# false.

attrs = []
attrs.push(:name.like('a%')) if name_starts_with_a?
attrs.push(:name.like('%z')) if name_ends_with_z?
# same as ds.filter if name_starts_with_a?
# and name_ends_with_z? are both false.

attrs = []
attrs.push('number > 10') if gt_10?
attrs.push('number < 30') if lt_30?
ds.filter(attrs.join(' AND '))
# same as ds.filter('') if gt_10? and lt_30?
# are both false.

Sequel now handles all of the above behaviors by just returning a clone of the receiving dataset. This is just another of the many good suggestions that are submitted via the Google Group. This particular suggestion came from Shawn.