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copied!<p>The safest way to handle migrations of this nature, Heroku or no, is to strictly adopt a compatibility approach with your schema and code: </p> <ul> <li>Every additive or transformative schema change must be backwards-compatible;</li> <li>Every destructive schema change must be performed <em>after</em> the code that depends on it has been removed;</li> <li>Every code change must either be: <ul> <li>durable against the possibility that associated schema changes have not yet been made (for instance, removing a model or a field on a model) <em>or</em></li> <li>made only after the associated schema change has been performed (adding a model or a field on a model)</li> </ul></li> </ul> <p>If you need to make a significant transformation of a model, this approach might require the following steps:</p> <ul> <li>Create a new database table to hold your new model structure, and deploy that migration</li> <li>Create a new model with the new structure, and code to copy changes from the old model to the new model when the old model changes, and deploy that code</li> <li>Execute a migration or code action to copy all old model data to the new model</li> <li>Update your codebase to use the new model rather than the old model, deleting the old model, and deploy that code</li> <li>Execute a migration to delete the old model structure from the database</li> </ul> <p>With some thought and planning, it can be used for more drastic changes as well: </p> <ul> <li>Deploy code that completely removes dependence on a section of the database, presumably replacing those sections of the site with maintenance pages</li> <li>Deploy a migration that makes drastic changes that would not for whatever reason work with the above dual-model workflow</li> <li>Deploy code that brings the affected sections back with the new model structure supported</li> </ul> <p>This can be hard to organize and requires strict discipline and firm understanding of your code's interaction with your database, but in practice, it <strong>does</strong> allow for most changes to be made with no more downtime than the server restart itself imposes. </p>

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