import: Add function to re-map foreign keys for ManyToMany fields.

This will be used while for any ManyToMany field which
is being imported.
We add an internal function which takes in the old ID list
of the ManyToMany field and return the new updated ID list.

zerver/lib/import_realm.py

@@ -274,6 +274,51 @@ def re_map_foreign_keys_internal(data_table: List[Record],
             else:
                 item[field_name] = new_id
 
+def re_map_foreign_keys_many_to_many(data: TableData,
+                                     table: TableName,
+                                     field_name: Field,
+                                     related_table: TableName,
+                                     verbose: bool=False) -> None:
+    """
+    We need to assign new ids to rows during the import/export
+    process.
+
+    The tricky part is making sure that foreign key references
+    are in sync with the new ids, and this wrapper function does
+    the re-mapping only for ManyToMany fields.
+    """
+    for item in data[table]:
+        old_id_list = item['field_name']
+        new_id_list = re_map_foreign_keys_many_to_many_internal(
+            table, field_name, related_table, old_id_list, verbose)
+        item[field_name] = new_id_list
+        del item[field_name]
+
+def re_map_foreign_keys_many_to_many_internal(table: TableName,
+                                              field_name: Field,
+                                              related_table: TableName,
+                                              old_id_list: List[int],
+                                              verbose: bool=False) -> List[int]:
+    """
+    This is an internal function for tables with ManyToMany fields,
+    which takes the old ID list of the ManyToMany relation and returns the
+    new updated ID list.
+    """
+    lookup_table = id_maps[related_table]
+    new_id_list = []
+    for old_id in old_id_list:
+        if old_id in lookup_table:
+            new_id = lookup_table[old_id]
+            if verbose:
+                logging.info('Remapping %s %s from %s to %s' % (table,
+                                                                field_name + '_id',
+                                                                old_id,
+                                                                new_id))
+        else:
+            new_id = old_id
+        new_id_list.append(new_id)
+    return new_id_list
+
 def fix_bitfield_keys(data: TableData, table: TableName, field_name: Field) -> None:
     for item in data[table]:
         item[field_name] = item[field_name + '_mask']