Geografic-Information-System/scripts/v.db.dropcolumn/v.db.dropcolumn.py

#!/usr/bin/env python

############################################################################
#
# MODULE:       v.db.dropcolumn
# AUTHOR(S):    Markus Neteler
#               Converted to Python by Glynn Clements
# PURPOSE:      interface to db.execute to drop a column from the 
#               attribute table connected to a given vector map
#               - Based on v.db.addcolumn
#               - with special trick for SQLite
# COPYRIGHT:    (C) 2007 by the GRASS Development Team
#
#               This program is free software under the GNU General Public
#               License (>=v2). Read the file COPYING that comes with GRASS
#               for details.
#
#############################################################################


#%module
#% description: Drops a column from the attribute table connected to a given vector map.
#% keywords: vector
#% keywords: database
#% keywords: attribute table
#%end

#%option G_OPT_V_MAP
#% key: map
#%end

#%option G_OPT_V_FIELD
#%end

#%option G_OPT_DB_COLUMN
#% description: Name of attribute column to drop
#% required: yes
#%end

import sys
import os
import string
import grass.script as grass

def main():
    map    = options['map']
    layer  = options['layer']
    column = options['column']
    
    mapset = grass.gisenv()['MAPSET']
    
    # does map exist in CURRENT mapset?
    if not grass.find_file(map, element = 'vector', mapset = mapset):
	grass.fatal(_("Vector map <%s> not found in current mapset") % map)
    
    f = grass.vector_layer_db(map, layer)
    
    table = f['table']
    keycol = f['key']
    database = f['database']
    driver = f['driver']
    
    if not table:
	grass.fatal(_("There is no table connected to the input vector map. "
                      "Unable to delete any column. Exiting."))
    
    if column == keycol:
	grass.fatal(_("Unable to delete <%s> column as it is needed to keep table <%s> "
                      "connected to the input vector map <%s>") % \
                        (column, table, map))
    
    if not grass.vector_columns(map, layer).has_key(column):
	grass.fatal(_("Column <%s> not found in table <%s>") % (column, table))
    
    if driver == "sqlite":
	# echo "Using special trick for SQLite"
	# http://www.sqlite.org/faq.html#q13
	colnames = []
	coltypes = []
	for f in grass.db_describe(table)['cols']:
	    if f[0] == column:
		continue
	    colnames.append(f[0])
	    coltypes.append("%s %s" % (f[0], f[1]))

	colnames = ", ".join(colnames)
	coltypes = ", ".join(coltypes)

	cmds = [
	    "BEGIN TRANSACTION",
	    "CREATE TEMPORARY TABLE ${table}_backup(${coldef})",
	    "INSERT INTO ${table}_backup SELECT ${colnames} FROM ${table}",
	    "DROP TABLE ${table}",
	    "CREATE TABLE ${table}(${coldef})",
	    "INSERT INTO ${table} SELECT ${colnames} FROM ${table}_backup",
	    "CREATE UNIQUE INDEX ${table}_cat ON ${table} (${keycol} )",
	    "DROP TABLE ${table}_backup",
	    "COMMIT"
	    ]
	tmpl = string.Template(';\n'.join(cmds))
	sql = tmpl.substitute(table = table, coldef = coltypes, colnames = colnames, keycol = keycol)
    else:
	sql = "ALTER TABLE %s DROP COLUMN %s" % (table, column)
    
    if grass.write_command('db.execute', input = '-', database = database, driver = driver,
			   stdin = sql) != 0:
	grass.fatal(_("Deleting column failed"))
    
    # write cmd history:
    grass.vector_history(map)
    
if __name__ == "__main__":
    options, flags = grass.parser()
    main()