On Wednesday, March 16, 2022 at 1:56:33 AM UTC-7 Tom Clerckx wrote: > I was a bit confused about the explanation of the fake_migrate option in > the web2py documentation. After some experimenting, I wrote up the > following information for myself, just sharing it here with you. Maybe > someone can merge this information in the web2py documentation, if you > think it contributes to making this topic more clear. > -- > > I have to [sigh] relearn this frequently. I am finally beginning to take to heart your last comment below.
> The explanation here assumes sqlite as a database, but I believe holds > also for other databases. > > When changing a model, e.g. adding a field there are two places where > information is updated. > > - In the database-metadata files. These files are located in the > database folder. For each table a metadata file is created. If you want to > inspect these files, you can inspect them by unpickling them > > > - In the database.sqlite file. This is the database itself.The new > field will be added to the sqlite schema > > The database parameter *migrate* (=True by default) pushes changes that > are made in your model back into the meta-data file and in the sqlite file. > Another option *fake_migrate* (=False by default) pushes changes that are > made in your model back into the meta-data file, *but not in the sqlite > database file.* > Note also that this option will only push new Fields back into the > meta-data file. *It does not remove fields from the metadata file that > you may have removed in your model!!* > > For each field described in your model (and thus also in the metadata > file), you should have a corresponding element in the sqlite schema. > If that is not the case, you can have one of two errors: > Error 1: > The field is present in the sqlite database, it also exists in your model > file, but for some reason it does not exist in the metadata file. Because > it is defined in the model and it is not yet present in the metadata file, > web2py wants to create it by doing a migrate. > This migrate, includes updating both the metadata file *and* the sqlite > schema. The problem however, is that in the sqlite schema, this field > already exists! > This will cause an error similar as: > *sqlite3.OperationalError: duplicate column name: blablabla* > Solution: > We can fix this by telling web2py to do a 'fake_migrate'. > This is done by adding the parameter *fake_migrate=True* in the table > definition. > By doing a fake migrate, web2py will only update the metadata file and NOT > the sqlite database. As such, the fields in the database and in the > metadata file will get back in sync. > *It is important* to remove the fake_migrate parameter again after the > data sources are back in sync! > > Error 2: > The field is in the metadata file, it also exists in your model file, but > for some reason it does not exist in the sqlite schema. Because it is > defined in the model and in the metadata file, web2py will try to load this > field from the database. > But because it is not defined in the database, you will get an error such > as: > *sqlite3.OperationalError: no such column: person.blablabla* > Solution 2: > In this case we cannot do the fake-migrate directly, because the > fake-migrate option can add the new fields to the metadata file, but it > won't push the field to the database. So what we should do is: > > 1. First remove again the field in question from our model. > 2. Set the fake_migrate=True option > 3. Remove the meta-data file (keep a backup of it elsewhere, just to > make sure) > 4. Reload the database (e.g. by doing a screen refresh in the page > where you use the table in question)... this will recreate the meta-data > file based on your current model. > 5. Set fake_migrate=False (or just remove it, as False is the default > value) > 6. Put back the field in question in the model > 7. Reload the database once more > 8. By reloading the database, web2py will now push the new field again > in the metadatafile and it will also update the sqlite schema. Now all > data > is back in sync. > > > Sidenote: Why you should not have fake_migrate=True in a normal situation > If you would leave fake_migrate=True, you would actually cause Error 2 > above to happen when you add a new field to your table, because in that > case the new field > is only committed to the metadata-file and not to the sqlite database. > /dps -- Resources: - http://web2py.com - http://web2py.com/book (Documentation) - http://github.com/web2py/web2py (Source code) - https://code.google.com/p/web2py/issues/list (Report Issues) --- You received this message because you are subscribed to the Google Groups "web2py-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to web2py+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/web2py/0f7b3090-5949-4478-ae69-57f4f72b12c5n%40googlegroups.com.
