Summary of commonly used commands for database migration in Django

Preface

In Django, database migration is an important process to ensure that the database structure is consistent with the Django model definition. The following are the commonly used database migration commands in Django:

1. python makemigrations

• Function: This command is used to generate a new migration file based on changes in the model file() of a Django project. These migration files are Python scripts that describe how to synchronize the structure of a database with the corresponding Django model.
• Use scenarios: After you make changes to the model (such as adding fields, modifying the field type, or removing fields), you need to run this command to generate the migration file.
• Notice: This command will not immediately apply these changes to the database, it just creates a migration file that you need to usemigrateCommand to apply these changes.

2. python sqlmigrate <app_name> <migration_name>

• Function: This command will output the SQL statement corresponding to the given migration without actually performing the migration. It is used to view the original SQL operations Django will perform on the database and is very useful for debugging and understanding migration behavior.
• parameter：
  • <app_name>: Application name, that is, your Django application name.
  • <migration_name>: The number or name of the migration file, for example0003_auto_20231001_1200。
• Use scenarios: Use this command when you want to see how a migration will affect the database structure but do not want to perform the migration immediately.

3. python migrate

• Function: This command is used to apply the migration file and make necessary changes to the database to match the model. It will look for all unapplied migration files and follow them inmigrationsSequential execution in the directory.
• Use scenarios: This command needs to be run when you have generated the migration file and want to apply these changes to the database.
• Notice: This command will actually modify the database structure, so make sure that the database has been backed up before running (if needed).

4. python showmigrations

• Function: This command is used to list the names and their status (applied or not applied).
• Use scenarios: You can use this command when you want to see which migrations have been applied to the database and which have not been applied.

Sample flow

Suppose you have a Django project and you want to add a new field to an existing model. Here is the complete migration process:

• Open your Django appdocument.
• Find the model you want to modify and add a new field to it.
• From the command line, go to your Django project directory.
• Runpython makemigrationsOrder. Django detects changes in the model and creates a new migration file.
• Runpython migrateOrder. Django applies all unapplied migrations, including the migration files you just created, and updates the database structure to include new fields.

By following the above steps and commands, you can easily perform database migration in Django and ensure that your database structure is consistent with your model definition.

Attachment: Possible situations and solutions for Django data migration failure

In Django projects, data migration is a command used to modify database structure and data. However, sometimes there may be situations where migration failures are encountered. Here are some common causes of migration failure and their solutions:

• Missing dependency module
  If an error message like "ModuleNotFoundError: No module named 'xxxx'" appears during the migration process, it means that Django cannot find the required module. Please check whether the module prompted in the error message exists. If the module does not exist, the corresponding Python module needs to be installed.
• Database connection issues
  If an error is reported when executing the migrations file, the error message is ": (1045, 'Access denied for user 'xxxx'@'localhost' (using password: YES)')", which may be due to incorrect database username or password, or the user does not have permission to access the database. Please check whether the database username and password are correct, or whether the user is authorized to access the database.
• Data table constraint issues
  If an error is reported when executing the migrations file, the error message is ": NOT NULL constraint failed: xxxxx", which may be because the data table field is empty, which violates the NOT NULL constraint. Please check whether the data table field is empty. If it is empty, you need to set a default value for the field or modify the code logic to ensure that the field has a value.
• The data table already exists
  If an error message like ": relation 'xxxx' already exists" appears when executing the migrate command, it means that the data table already exists in the database. Please check whether the data table already exists in the database. If it exists, you need to manually delete the table.
• Circular dependency problem
  If an error is reported when executing migrations file, the error message is "Circular dependency detected", indicating that there is a circular dependency relationship. Please check the dependencies between models and try to split the dependencies into smaller parts.
• Data length exceeds limit
  If an error is reported when executing migrations file, the error message is ": value too long for type", which means that the data length exceeds the maximum length of the database field. Please modify the data length or modify the maximum length of the database field.
• Database lock or transaction failure
  If an error is reported when executing migrations file, the error message is ": database is locked" or ": current transaction is aborted", which may be due to the database being locked or the transaction fails. Please wait for other processes to complete the operation of the database, or restart the database.
• Unapplied migrations
  If an error message like "No migrations to apply" appears when executing the migrate command, it means there is no applicable migration. Please check if there is an unapplied migration file, if not, you need to create it.
• The database table does not exist
  If an error is reported when executing the migrations file, the error message is ": no such table", which means that there is no corresponding table in the database. Please run the migrate command to create the table. Similarly, if an error is reported when executing the migrations file, the error message is ": column does not exist", indicating that there is no corresponding column in the database. You also need to run the migrate command to create the column.
• Violation of uniqueness constraints
  If an error is reported when executing migrations file, the error message is ": UNIQUE constraint failed", indicating that the uniqueness constraint is violated. Please check whether there are duplicates in the data, or modify the uniqueness constraint.
  Summary: There are many reasons for the failure of Django data migration, and it needs to be analyzed specifically based on the error information. When encountering migration failure, you must first read the error message carefully to understand the reasons for the failure. Then, according to the reasons for the error, such as installing missing modules, checking database connections, modifying data table structure, processing circular dependencies, adjusting data length, waiting for database unlocking, rolling back transactions, creating migration files, etc. In the process of solving migration failures, it is necessary to pay attention to the standardization of the code and the consistency of the data to ensure the stability and reliability of the Django project.

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