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replace just the changed fields in the model #187

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legrostdg opened this Issue Nov 27, 2013 · 4 comments

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As discussed in the mailing list, it would be cool to have a kind of "diffToUpdates" in persistent which would only update fields (or return a list of Update's) which differ between two models:

For example:

data Person = Person { name :: String, age :: Int }

oldModel = Person "Michael" 26

newModel = Person "Michael" 27

"diffToUpdates oldModel newModel" would give "[PersonAge =. 27]"

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gregwebs commented Aug 29, 2014

Note that updateGet is another way of getting at this: apply updates to the db and get back the newModel.

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gregwebs commented Sep 23, 2014

I think I would call such a function overwriteUpdates. Although updateGet is still preferable, this is needed to merge 2 existing records

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gregwebs commented Sep 23, 2014

overwriteUpdates :: (PersistEntity record, Eq record) => record -> record -> [Update record]
overwriteUpdates record1 record2 = catMaybes $ zipWith overwrite fields1 fields2
  where
    overwrite up1@(field1 =. pf1) up2@(field2 =. pf2) = if toPersistValue pf1 == toPersistValue pf2 then Nothing else Just up2
    fields1 = toUpdates record1
    fields2 = toUpdates record2
    entity = entityDef $ Just record1

The only problem is that toUpdates does not exist! As far as I can tell it needs to be generated in template-haskell for each record, because EntityField is essentially a GADT that we can't just automatically derive Enum and Bounded for.

This feature can be added without breaking backwards compatibility.

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snoyberg commented Jul 19, 2015

Closing out old issues, please reopen if still relevant

@snoyberg snoyberg closed this Jul 19, 2015

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