Add convertdb command.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

HISTORY.rst

@@ -1,4 +1,9 @@
-3.0.1 (208-07-08)
+3.1 (2018-XX-XX)
+================
+- Improve database vendor migration support
+- Add convertdb management command.
+
+3.0.1 (2018-07-08)
 =================
 - Pin javascript libraries to specific versions to avoid using 
   potentianlly broken updates automatically. GitLab issue #486.

mayan/apps/common/management/commands/convertdb.py

@@ -0,0 +1,86 @@
+from __future__ import unicode_literals
+
+import errno
+import os
+
+from pathlib import Path
+
+from django.conf import settings
+from django.core import management
+from django.core.management.base import CommandError
+from django.utils.encoding import force_text
+from django.utils.translation import ugettext_lazy as _
+
+from common.utils import fs_cleanup
+from documents.models import DocumentType
+
+CONVERTDB_FOLDER = 'convertdb'
+CONVERTDB_OUTPUT_FILENAME = 'migrate.json'
+
+
+class Command(management.BaseCommand):
+    help = 'Convert from a database backend to another one.'
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            '--from', action='store', default='default', dest='from',
+            help=_(
+                'The database from which data will be exported. If omitted '
+                'the database named "default" will be used.'
+            ),
+        )
+        parser.add_argument(
+            '--to', action='store', default='default', dest='to',
+            help=_(
+                'The database to which data will be imported. If omitted '
+                'the database named "default" will be used.'
+            ),
+        )
+        parser.add_argument(
+            '--force', action='store_true', dest='force',
+            help=_(
+                'Force the conversion of the database even if the receving '
+                'database is not empty.'
+            ),
+        )
+
+    def handle(self, *args, **options):
+        # Create the media/convertdb folder
+        convertdb_folder_path = force_text(
+            Path(
+                settings.MEDIA_ROOT, CONVERTDB_FOLDER
+            )
+        )
+
+        try:
+            os.makedirs(convertdb_folder_path)
+        except OSError as exception:
+            if exception.errno == errno.EEXIST:
+                pass
+
+        convertdb_file_path = force_text(
+            Path(
+                convertdb_folder_path, CONVERTDB_OUTPUT_FILENAME
+            )
+        )
+
+        management.call_command(
+            'dumpdata', all=True, database=options['from'],
+            natural_primary=True, natural_foreign=True,
+            output=convertdb_file_path, interactive=False,
+            indent=4,
+        )
+
+        if DocumentType.objects.using('default').count() and not options['force']:
+            fs_cleanup(convertdb_file_path)
+            raise CommandError(
+                'There is existing data in the database that will be '
+                'used for the import. If you proceed with the conversion '
+                'you might lose data. Please check you settings.'
+            )
+
+        management.call_command(
+            'loaddata', convertdb_file_path, database=options['to'],
+            interactive=False
+        )
+        fs_cleanup(convertdb_file_path)