Avoid exception when changing fieldname from upper to lowercase (#2517)

frappe/model/db_schema.py

@@ -254,7 +254,7 @@ class DbTable:
 	def get_columns_from_db(self):
 		self.show_columns = frappe.db.sql("desc `%s`" % self.name)
 		for c in self.show_columns:
-			self.current_columns[c[0]] = {'name': c[0],
+			self.current_columns[c[0].lower()] = {'name': c[0],
 				'type':c[1], 'index':c[3]=="MUL", 'default':c[4], "unique":c[3]=="UNI"}
 
 	# GET foreign keys
@@ -290,7 +290,7 @@ class DbTable:
 
 	def alter(self):
 		for col in self.columns.values():
-			col.build_for_alter_table(self.current_columns.get(col.fieldname, None))
+			col.build_for_alter_table(self.current_columns.get(col.fieldname.lower(), None))
 
 		query = []
 
@@ -298,7 +298,8 @@ class DbTable:
 			query.append("add column `{}` {}".format(col.fieldname, col.get_definition()))
 
 		for col in self.change_type:
-			query.append("change `{}` `{}` {}".format(col.fieldname, col.fieldname, col.get_definition()))
+			current_def = self.current_columns.get(col.fieldname.lower(), None)
+			query.append("change `{}` `{}` {}".format(current_def["name"], col.fieldname, col.get_definition()))
 
 		for col in self.add_index:
 			# if index key not exists
@@ -395,7 +396,8 @@ class DbColumn:
 			return
 
 		# type
-		if (current_def['type'] != column_def) or \
+		if (current_def['type'] != column_def) or\
+			self.fieldname != current_def['name'] or\
 			((self.unique and not current_def['unique']) and column_def not in ('text', 'longtext')):
 			self.table.change_type.append(self)