I would additionally make each migration function only go from one version
to the next version up. Then chain them together up to the latest version.
That way you only have to make 1 migration function for each version bump
and leave the others untouched and still upgrade from any previous version.
I think you'd have to have an intermediate format for the migration... like
first decode whatever was stored into a Dict String Value. Then only
decode, modify, and re-encode the changed model parts in each migration
method. Then at the end, take the Dict String Value and decode that into
the current model.
loadModel : String -> Dict String Value -> Result String Model
loadModel dataVersion d =
case dataVersion of
"0.0.1" ->
Dict.insert "aNewProperty" (Encoder.string "A Default Value") d
|> loadModel "0.0.2"
"0.0.2" ->
Dict.remove "anOldProperty" d
|> loadModel "0.0.3"
// at latest version
"0.0.3" ->
decodeModelFromDict d
_ ->
Result.Err "Invalid Version"
This is just an idea. I haven't tried it.
On Tuesday, November 8, 2016 at 9:20:56 AM UTC-6, Peter Damoc wrote:
>
> You could load the raw data into the app using Value instead of whatever
> you have now and create a series of Json Decoders for each version of the
> schema.
> You could then use Json.Decode.oneOf to produce the current structure
> from any of the old schemas.
>
> You could also use a schema version field on the data and then do the
> decoding in two parts using Json.Decod.andThen
>
>
>
>
> On Tue, Nov 8, 2016 at 5:02 PM, Jacky See <[email protected] <javascript:>
> > wrote:
>
>> I have an Elm app that would store the entire model into the local
>> storage, using `programWithFlags` to feed in the data directly when `init`.
>> As the app develop, more fields is added to the model so there will be
>> model mismatch between localStorage and the new model and Elm would fail at
>> the JS side when trying to init.
>> Obviously, there should be some migration plan here at the js side to
>> update the stored model to the new shape.
>> Personally, I wish I could have something like Json.Decode to handle the
>> model directly in Elm, maybe peform a task?
>> What would you guys recommend to do?
>>
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>
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> blog: http://damoc.ro/
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