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Add README section on cross-table migrations

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fabianlindfors 2023-11-22 00:15:21 +01:00
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@ -34,6 +34,7 @@ Designed for Postgres 12 and later.
- [Create enum](#create-enum)
- [Remove enum](#remove-enum)
- [Custom](#custom)
- [Complex changes across tables](#complex-changes-across-tables)
- [Commands and options](#commands-and-options)
- [`reshape migration start`](#reshape-migration-start)
- [`reshape migration complete`](#reshape-migration-complete)
@ -284,7 +285,7 @@ foreign_key = "items_user_id_fkey"
#### Add column
The `add_column` action will add a new column to an existing table. You can optionally provide an `up` setting. This should be an SQL expression which will be run for all existing rows to backfill the new column.
The `add_column` action will add a new column to an existing table. You can optionally provide an `up` setting. This should be an SQL expression which will be run for all existing rows to backfill the new column. `up` may also reference another table to perform cross-table migrations (see ["Complex changes across tables"](#complex-changes-across-tables)).
_Example: add a new column `reference` to table `products`_
@ -448,7 +449,7 @@ column = "created_at"
#### Remove column
The `remove_column` action will remove an existing column from a table. You can optionally provide a `down` setting. This should be an SQL expression which will be used to determine values for the old schema when inserting or updating rows using the new schema. The `down` setting must be provided when the removed column is `NOT NULL` or doesn't have a default value.
The `remove_column` action will remove an existing column from a table. You can optionally provide a `down` setting. This should be an SQL expression which will be used to determine values for the old schema when inserting or updating rows using the new schema. `down` may also reference another table to perform cross-table migrations (see ["Complex changes across tables"](#complex-changes-across-tables)) . The `down` setting must be provided when the removed column is `NOT NULL` or doesn't have a default value.
Any indices that cover the column will be removed.
@ -596,6 +597,40 @@ abort = """
"""
```
### Complex changes across tables
The `up` and `down` options available for many actions can also perform more complex changes that span tables.
_Example: move `email` column from `users` to `profiles` table_
```toml
[[actions]]
type = "remove_column"
table = "users"
column = "email"
# When `profiles` is changed in the new schema, we write the email address back to the removed column
[actions.down]
table = "profiles"
value = "email"
where = "id = user_id"
[[actions]]
type = "add_column"
table = "profiles"
[actions.column]
name = "email"
type = "TEXT"
nullable = false
# When `users` is updated in the old schema, we write the email value to `profiles`
[actions.up]
table = "users"
value = "email"
where = "user_id = id"
```
## Commands and options
### `reshape migration start`