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Contents
module Bmg
module Operator
#
# Project operator.
#
# Projects operand's tuples on given attributes, that is, keep those attributes
# only. The operator takes care of removing duplicates.
#
# Example:
#
# [{ a: 1, b: 2 }] project [:b] => [{ b: 2 }]
#
# All attributes in the attrlist SHOULD be existing attributes of the
# input tuples.
#
class Project
include Operator::Unary
def initialize(type, operand, attrlist)
@type = type
@operand = operand
@attrlist = attrlist
end
private
attr_reader :attrlist
public
def each
seen = {}
@operand.each do |tuple|
projected = project(tuple)
unless seen.has_key?(projected)
yield(projected)
seen[projected] = true
end
end
end
def insert(arg)
case arg
when Hash then operand.insert(valid_tuple!(arg))
when Enumerable then operand.insert(arg.map{|t| valid_tuple!(t) })
else
super
end
end
def update(tuple)
operand.update(valid_tuple!(tuple))
end
def delete
operand.delete
end
def to_ast
[ :project, operand.to_ast, attrlist ]
end
protected ### optimization
def _restrict(type, predicate)
operand.restrict(predicate).project(attrlist)
end
protected ### inspect
def args
[ attrlist ]
end
private
def project(tuple)
tuple.delete_if{|k,_| !@attrlist.include?(k) }
end
def valid_tuple!(tuple)
offending = tuple.keys - attrlist
raise InvalidUpdateError, "#{offending.inspect} cannot be updated" unless offending.empty?
tuple
end
end # class Project
end # module Operator
end # module Bmg
Version data entries
13 entries across 13 versions & 1 rubygems