yesodweb / yesodweb/persistent
Can Persistent filter with a comparison on two fields in the same model?
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- Dominant language
- Haskell
- Stars
- 486
- Forks
- 306
- PR merge metrics
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Description
Hello,
Thank you for such a great library!
I believe I've run into a limitation in persistent, but maybe it's already possible to do this and I'm overlooking how to do it?
I have a model like the following:
Thing
...
fooVal Int
barVal Int
and I'd like to use selectList with a filter that compares those values such as:
selectThings = runDB $ selectList [ThingFooVal >. ThingBarVal] []
This fails with a type error like:
Couldn't match type 'EntityField Thing Int' with 'Int' arising from a use of ThingBarVal
Is this not possible in persistent directly, or is there a different way I should setup the query?
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reviewing Persistent's selectList filter API and the ThingFooVal >. ThingBarVal example to understand how field comparisons are represented. Done means determining whether same-model field comparisons are supported and documenting the supported query path or confirmed limitation.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- haskell
- Domain
- databases
- Issue type
- Feature
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100