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To transform a string column into an array column in Django PostgreSQL migration, follow these steps:

1. Create a new migration file using the makemigrations command: python manage.py makemigrations your_app_name --empty your_migration_name
2. In the new migration file, import the PostgreSQL operation from django.contrib.postgres.operations. from django.db import migrations from django.contrib.postgres.operations import PostgreSQL

3. Define two operations in the Migration class: RunSQL and PostgreSQL.

   class Migration(migrations.Migration):
   
       operations = [
           migrations.RunSQL("ALTER TABLE your_table_name ADD COLUMN new_column_name text[];"),
           PostgreSQL("UPDATE your_table_name SET new_column_name = ARRAY[column_name];"),
           migrations.AlterField(
               model_name='your_model_name',
               name='new_column_name',
               field=ArrayField(models.CharField(max_length=your_max_length)),
           ),
       ]
   

4. In the RunSQL operation, add a SQL statement to create a new array column in the table. migrations.RunSQL("ALTER TABLE your_table_name ADD COLUMN new_column_name text[];")
5. In the PostgreSQL operation, add a SQL statement to update the values of the new array column from the string column. PostgreSQL("UPDATE your_table_name SET new_column_name = ARRAY[column_name];")
6. In the AlterField operation, add a new array field to the model, and set the max_length parameter to the maximum length of the strings in the original column. migrations.AlterField( model_name='your_model_name', name='new_column_name', field=ArrayField(models.CharField(max_length=your_max_length)), )
7. Run the migration using the migrate command: python manage.py migrate your_app_name Now, the string column should have been transformed into an array column.