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Because we operate in an environment where new and old code are running simultaneously against the same database, new code must always be compatible with the older database schema. Newer deployments can add tables and columns, but neither can be deleted unless the old code is no longer referencing the deleted tables or columns.
How to drop a column
For either a nullable or non-nullable column, first make sure there are no other models or code that actually use the column. If there are, make adjustments to those first before working through the deletion flow.
For NULLABLE columns, this is involves TWO releases:
- Remove all usages of the column, including updating the model to not refer to the field/column
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- anymore (i.e. Model field must be removed in this step)
- Drop the column (with a migration).
For NOT-NULLABLE columns, this involves THREE releases:
- Update the model and generate a migration making the column nullable (`null=True`)
- Remove all usages of the column, including updating the model to not refer to the field/column anymore (i.e. Model field must be removed in this step)
- Drop the column (with a migration).
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When using AWS Aurora, a nullable column can be added to existing large (100k rows+?) tables without causing downtime. However, the migration may still timeout in GoCD - so please coordinate the release with DevOps.
- Make a new field addition in model with null=True.
- Generate the model-change migrations locally.
- Create a pull request containing the model change and newly-generated migrations.
- Merge the migration pull request.
- The release process will run the migration and add the nullable column to table.
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On AWS Aurora, indexes can be build on large tables without causing downtime, but this requires devops coordination as the migration may timeout in GoCD.
- Add the new index on model fields.
- Generate the model-change migrations locally.
- Make pull request containing the model change and newly-generated migrations.
- Merge the migration pull request.
- Release will run the migration and add the index to table.
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