Database Migrations

A project often undergoes changes related to database schema during course of its development. It may also require patching of existing data. Frappe comes with a migration and patch system tools to handle these scenarios.

When there are schema changes in your app, to migrate your existing site's database to the new schema, you should run the command.

bench --site [sitename] migrate

Schema changes

You can edit a DocType to add, remove or change fields. On saving a DocType, a JSON file containing the DocType data is added to source tree of your app. When you add an app to a site, the DocTypes are installed (database tables are created) using this JSON file.

For making schema changes, you must enable Developer Mode.

On running a migrate, all apps migrate to their current version. First, we run the "before_migrate" hook to sync user permissions. Then the actual patches are run, which migrate the various apps. After this, we sync the following components:

  • Database Schema
  • Background Jobs
  • Fixtures [Read more]
  • Dashboards, Desktop Icons and Web Pages
  • Updates Translations
  • Rebuild Search Index for all routes

Particularly for DocTypes, we compare the MD5 hash of each DocType JSON with the hashes we have stored in the DocType database table. If the hashes don't match, we reload the particular DocType. This technique is called checksum comparison, you can learn more about it here. Finally, we run a "after_migrate" hook to finish the migration. You can find the commands run for the individual processes at bench-site-commands

Note: Up Until v13 DocTypes were synced based on the modified timestamps in the JSON file. This method was error-prone so we moved to the more robust hash comparison method. You can read more about it here

When you remove or rename fields in the DocType, the corresponding database columns are not removed from the database table, but they will not be visible in the form view. This is done to avoid any potential data loss situations and to allow you write related data migrations (patches) which might need values from old fields.

Frappe doesn't support reverse schema migrations.

Data Migrations

On introducing data related changes, you might want to run one off scripts to change existing data to match expectations as per new code. We call these scripts patch in frappe.

Writing a patch

To write a patch, you must write an execute method in a python script and add it to patches.txt of your app.

It is recommended to make a file with a patch number and name in its path and add it to a patches package (directory) in your app. You can then add a line with dotted path to the patch module to patches.txt.

The directory structure followed in Frappe is as below

└── patches
    └── v12_0

The patch can then be added to patches.txt by its dotted path.


Schema during patch

The DocType meta available in the execute function will be as per the old JSON. This is so that you can write migration code assuming you still have the old fields. After the patch is run, the new schema is applied to the DocType.

If you want to have the new schema during your patch execution, use the reload_doc method.

import frappe

def execute():
    frappe.reload_doc(module_name, "doctype", doctype_name)

    # your patch code here

Post-Model sync patches [v14 / develop branch only]

Often your patch might not require access to database schema before DocType models are synced with database. In such cases it's better to keep the patch in [post_model_sync] section of patches.txt.

patches.txt supports INI-like file format where two sections specify when a patch should run - before or after doctype schema migration. Post model sync patches do not require reloading any doctypes as all doctypes are reloaded before executing them. Here is example of such patches.txt file:



One off Python statements

You can also add one off python statements in patches.txt using the syntax,

execute:frappe.delete_doc('Page', 'applications', ignore_missing=True)

Patch execution order

Patches run in the order they are defined. All lines in patches.txt have to be unique. If a patch has been run before, it won't run again. If you want to run a patch again, add a comment that will make the line appear as new.

For Example,

frappe.patches.v12_0.my_awesome_patch #2019-09-08

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