feat: Labels reordering

Closes #289

server/utils/migrations.js

@@ -0,0 +1,49 @@
+const POSITION_GAP = 65535;
+
+const addPosition = async (knex, tableName, parentFieldName) => {
+  await knex.schema.table(tableName, (table) => {
+    /* Columns */
+
+    table.specificType('position', 'double precision');
+
+    /* Indexes */
+
+    table.index('position');
+  });
+
+  const records = await knex(tableName).orderBy([parentFieldName, 'id']);
+
+  let prevParentId;
+  let position;
+
+  // eslint-disable-next-line no-restricted-syntax
+  for (record of records) {
+    if (record[parentFieldName] === prevParentId) {
+      position += POSITION_GAP;
+    } else {
+      prevParentId = record[parentFieldName];
+      position = POSITION_GAP;
+    }
+
+    // eslint-disable-next-line no-await-in-loop
+    await knex(tableName)
+      .update({
+        position,
+      })
+      .where('id', record.id);
+  }
+
+  return knex.schema.table(tableName, (table) => {
+    table.dropNullable('position');
+  });
+};
+
+const removePosition = (knex, tableName) =>
+  knex.schema.table(tableName, (table) => {
+    table.dropColumn('position');
+  });
+
+module.exports = {
+  addPosition,
+  removePosition,
+};