persistent-mtl
Monad transformer for the persistent API
https://github.com/brandonchinn178/persistent-mtl#readme
Version on this page: | 0.2.2.0 |
LTS Haskell 23.0: | 0.5.1@rev:1 |
Stackage Nightly 2024-12-13: | 0.5.1@rev:1 |
Latest on Hackage: | 0.5.1@rev:1 |
persistent-mtl-0.2.2.0@sha256:63702f9b4ed7376393f3a6a7e615c9932fc2f70509722bd1d008216a13f7b80b,2725
Module documentation for 0.2.2.0
- Control
- Control.Monad
- Control.Monad.IO
- Control.Monad
- Database
persistent-mtl
Use the persistent
API in your monad transformer stack, seamlessly interleaving business logic with database operations by simply dropping SqlQueryT
into your stack.
Features:
- Easy integration into a monad transformer stack
- Monad type class to generalize functions that use database operations
- Simple transaction control
- Supports mocking database operations in tests
Quickstart
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
import Control.Monad.IO.Class (MonadIO(..))
import Control.Monad.Logger (runStderrLoggingT)
import Database.Persist.Sql (Entity(..), toSqlKey, (<.))
import Database.Persist.Monad
import Database.Persist.Sqlite (withSqlitePool)
import Database.Persist.TH
import UnliftIO (MonadUnliftIO(..), wrappedWithRunInIO)
import Database.Persist.Monad.TestUtils (runMockSqlQueryT, withRecord)
import Test.Tasty.HUnit (Assertion, (@?=))
share [mkPersist sqlSettings, mkMigrate "migrate"] [persistLowerCase|
Person
name String
age Int
deriving Show Eq
|]
newtype MyApp a = MyApp
{ unMyApp :: SqlQueryT IO a
}
deriving (Functor, Applicative, Monad, MonadIO, MonadSqlQuery)
instance MonadUnliftIO MyApp where
withRunInIO = wrappedWithRunInIO MyApp unMyApp
getYoungPeople :: MonadSqlQuery m => m [Entity Person]
getYoungPeople = selectList [PersonAge <. 18] []
main :: IO ()
main = runStderrLoggingT $ withSqlitePool "db.sqlite" 5 $ \pool ->
liftIO $ runSqlQueryT pool $ unMyApp $ do
runMigration migrate
insert_ $ Person "Alice" 25
insert_ $ Person "Bob" 10
youngsters <- getYoungPeople
liftIO $ print youngsters
-- unit test with mocks!
unit_my_function :: Assertion
unit_my_function = do
let person1 = Entity (toSqlKey 1) (Person "Child1" 10)
result <- runMockSqlQueryT getYoungPeople
[ withRecord @Person $ \case
SelectList _ _ -> Just [person1]
_ -> Nothing
]
result @?= [person1]
What’s wrong with just using persistent
?
Using persistent
in production code
persistent
runs all of its functions in SqlPersistT
, which is an alias for ReaderT SqlBackend
. Since all functions run in this concrete monad and not a generalized type class, it becomes difficult to integrate database operations into your monad transformer stack. Below are some examples of trying to integrate persistent
functions into a monad transformer application, and the drawbacks of each option.
Option 1: Add SqlPersistT
to your monad transformer stack
One might look at the SqlPersistT
type and think it’s a monad transformer, and add it to their monad transformer stack. But since persistent
functions run in the concrete SqlPersistT
monad (and not with a type class), you’ll need some way of lifting SqlPersistT
into your application monad.
Before going further, I do want to point out that SqlBackend
represents a single database connection, so adding SqlPersistT
to your monad transformer stack would run your entire application in a single connection (read: single transaction)! So for most applications, this option probably won’t work for you, but let’s assume you have a use-case where this isn’t an issue.
Option 1a is to write liftSqlPersist
specifically for your application monad:
newtype MyApp a = MyApp (ReaderT MyAppConfig (SqlPersistT (LoggingT IO)) a)
-- Notice the duplication here: anything inside `SqlPersistT` in your stack
-- needs to go in here.
liftSqlPersist :: SqlPersistT (LoggingT IO) a -> MyApp a
liftSqlPersist = MyApp . lift
But then any function that runs database connections is taken out of mtl-style add needs to be concretely typed to MyApp
-- you originally had a nice mtl-style function with a generalized monad
foo :: MonadReader MyAppConfig m => m ()
foo = do
config <- ask
_ <- bar config
return ()
-- but adding a database operation forces us to remove the generalization
foo :: MyApp ()
foo = do
config <- ask
_ <- bar config
_ <- liftSqlPersist $ get $ configUserId config
return ()
So then you might try option 1b and write a type class that will lift SqlPersistT
:
class MonadLiftSqlPersist m where
-- Remember how we had to duplicate anything inside `SqlPersistT` in your
-- stack? The stack within `SqlPersistT` can be different between monads, so
-- you need to define the inner type for each monad
type Inner m :: Type -> Type
liftSqlPersist :: SqlPersistT (Inner m) a -> m a
instance MonadLiftSqlPersist MyApp where
type Inner MyApp = LoggingT IO
liftSqlPersist = MyApp . lift
which still has the unfortunate problem of copy-pasting whatever is inside SqlPersistT
into the Inner
type family instance.
But the main problem with both of these options is that liftSqlPersist
will only contain the context you put inside SqlPersistT
, meaning that within a liftSqlPersist
action, you can’t get access to MyAppConfig
! Of course, you could always make SqlPersistT
the very first monad transformer in your stack, but that might not work in another situation. Plus, you’d have even more monad transformers to copy into the type of liftSqlPersist
.
Option 2: Manually run runSqlPool
every time you run a persistent
function
Here, you might store the Pool SqlBackend
in your monad transformer stack and then use runSqlPool
to immediately unwrap SqlPersistT
.
data MyAppConfig = MyAppConfig
{ backendPool :: Pool SqlBackend
, ...
}
runQuery :: MonadReader MyAppConfig m => SqlPersistT m a -> m a
runQuery m = do
MyAppConfig{backendPool} <- ask
runSqlPool m backendPool
foo :: MonadReader MyAppConfig m => m ()
foo = do
config <- ask
_ <- bar config
_ <- runQuery $ get $ configUserId config
return ()
Great! Let me first say that this is not a bad solution. You could even make your own type class like MonadHasBackendPool
to abstract away monads that contain a Pool SqlBackend
, not necessarily the whole MyAppConfig
.
There are two drawbacks with this approach, one minor drawback and one major drawback. The minor drawback is that you have to put the Pool SqlBackend
into your environment yourself. It would be great if there could be a monad transformer and type class already made for you to easily plug it in. It’s not that much code, so this isn’t a big deal, but if you’re quickly bootstrapping a new project with persistent, it’d be nice to reach for something already built.
The major drawback with this approach is transactions and composability. runSqlPool
(and runQuery
in this example) runs its action within a single transaction. Say you have two functions that run separate, composable actions that interleave business logic and database operations:
foo :: MonadReader MyAppConfig m => m ()
foo = do
-- business logic
runQuery $ insert_ $ ...
-- more business logic
bar :: MonadReader MyAppConfig m => m ()
bar = do
-- business logic
runQuery $ insert_ $ ...
-- more business logic
There is no way to compose foo
and bar
so that it all runs within a single database transaction. You could try
fooAndBar :: MonadReader MyAppConfig m => m ()
fooAndBar = runQuery $ do
lift foo
-- something else
lift bar
but foo
and bar
each run their own runQuery
function, so actually, fooAndBar
uses three connections (i.e. three transactions): one connection from runQuery
in fooAndBar
and one connection each from foo
and bar
.
Option 3: persistent-mtl
So what does persistent-mtl
do differently?
-
It stores the entire
Pool SqlBackend
inSqlQueryT
, which means you can addSqlQueryT
to your monad transformer stack. Remember that the problem with addingSqlPersistT
to your monad transformer stack is that your entire application would run with a single database connection, aka a single database transaction. -
It provides a
MonadSqlQuery
type class out of the box and all ofpersistent
’s functions lifted to useMonadSqlQuery
-
It provides a
withTransaction
function that runs the given action within a single transaction. For example,foo :: MonadSqlQuery m => m () foo = do -- business logic insert_ $ ... -- more business logic bar :: MonadSqlQuery m => m () bar = do -- business logic insert_ $ ... -- more business logic fooAndBar :: MonadSqlQuery m => m () fooAndBar = withTransaction $ do foo -- something else bar
fooAndBar
will run bothfoo
andbar
in the same transaction. Note thatfoo
andbar
themselves don’t say anything about transactions. By default, using apersistent
function withoutwithTransaction
will run each query in its own transaction. And iffoo
did usewithTransaction
, it would start a transaction within a transaction (if the SQL backend supports it). Now,foo
andbar
are composable!
In summary, persistent-mtl
takes all the good things about option 2, implements them out of the box (so you don’t have to do it yourself), and makes your business logic functions composable with transactions behaving the way YOU want.
Easy transaction management
Some databases will throw an error if two transactions conflict (e.g. PostgreSQL). The client is expected to retry transactions if this error is thrown. persistent
doesn’t easily support this out of the box, but persistent-mtl
does!
import Database.PostgreSQL.Simple.Errors (isSerializationError)
main :: IO ()
main = withPostgresqlPool "..." 5 $ \pool -> do
let env = mkSqlQueryEnv pool $ \env -> env
{ retryIf = isSerializationError . fromException
, retryLimit = 100 -- defaults to 10
}
-- in any of the marked transactions below, if someone else is querying
-- the postgresql database at the same time with queries that conflict
-- with yours, your operations will automatically be retried
runSqlQueryTWith env $ do
-- transaction 1
insert_ $ ...
-- transaction 2
withTransaction $ do
insert_ $ ...
-- transaction 2.5: transaction-within-a-transaction is supported in PostgreSQL
withTransaction $ do
insert_ $ ...
insert_ $ ...
-- transaction 3
insert_ $ ...
Because of this built-in retry support, any IO actions inside withTransaction
have to be explicitly marked with rerunnableIO
. If you try to use a function with a MonadIO m
constraint, you’ll get a compile-time error!
.../Foo.hs:100:5: error:
• Cannot run arbitrary IO actions within a transaction. If the IO action is rerunnable, use rerunnableIO
• In a stmt of a 'do' block: arbitraryIO
In the second argument of ‘($)’, namely
‘withTransaction
$ do insert_ record1
arbitraryIO
insert_ record2’
|
100 | arbitraryIO
| ^^^^^^^^^^^
Note that this only applies for transactions, so MonadIO
and MonadSqlQuery
constraints can still co-exist (for a function with IO actions that are not rerunnable) as long as the function is never called within withTransaction
.
Testing functions that use persistent
operations
Generally, I would recommend someone using persistent
in their application to make a monad type class containing the API for their domain, like
class MonadAppService m where
getYoungPeople :: m [Entity Person]
instance MonadAppService MyApp where
getYoungPeople = selectList [PersonAge <. 18] []
so that writing unit tests would mock out domain-level abstractions. I generally wouldn’t recommend mocking out the entire database state; if you’re testing complex database queries, you should just write integration tests and check that the queries do what you expect on an actual database.
But maybe you have a small function that uses selectList
and it’s not worth making a whole type class to wrap that call. With persistent
, selectList
runs a SqlPersistT
action, which is completely un-introspectable. Sure, you could pass in a SqlBackend
that intercepts all queries, but you’d be mocking extremely low level behavior — your mock would need to know the exact SELECT
query selectList
sends.
persistent-mtl
, on the other hand, provides MockSqlQueryT
which you can use to execute your MonadSqlQuery
functions with a list of mocks, where a mock intercepts SqlQueryRep
, a data representation of each persistent
function, and returns the result. For example, to mock selectList
, you’d simply do
runMockSqlQueryT getYoungPeople
[ withRecord @Person $ \case
SelectList _ _ -> Just mockedPersonList
_ -> Nothing
]
and MockSqlQueryT
would intercept a selectList
call for a Person
record and return your mocked result. Each persistent
function has a corresponding data type constructor (with a few exceptions, such as selectSource
, which works differently).
If your function does some complex raw SQL queries, you can intercept those like this:
crazyFunction :: MonadSqlQuery m => String -> m [Int]
crazyFunction postTitle = rawSql
"SELECT age FROM person INNER JOIN post ON person.id = post.author WHERE post.title = ?"
[toPersistValue postTitle]
let mockRawSql = mockQuery $ \case
RawSql _ [toPersistValue "foo"] -> Just [1]
RawSql _ [toPersistValue "bar"] -> Just [2]
_ -> Nothing
-- returns [1]
runMockSqlQueryT (crazyFunction "foo") [mockRawSql]
-- returns [2]
runMockSqlQueryT (crazyFunction "bar") [mockRawSql]
-- error: Could not find mock for query
runMockSqlQueryT (crazyFunction "baz") [mockRawSql]
Changes
Unreleased
0.2.2.0
- Fix for persistent 2.13
0.2.1.0
- Add
rerunnableLift
forSqlTransaction
- Use
unliftio-pool
instead ofresourcet-pool
, which has better async exeception safety
0.2.0.0
- Use a separate monad within
withTransaction
to prevent unsafe/arbitrary IO actions (#7, #28) - Add
MonadRerunnableIO
to support IO actions withinwithTransaction
only if the IO action is determined to be rerunnable - Add built-in support for retrying transactions if a serialization error occurs
- Remove
SqlQueryRep
as an export fromDatabase.Persist.Monad
. You shouldn’t ever need it for normal usage. It is now re-exported byDatabase.Persist.Monad.TestUtils
, since most of the usage ofSqlQueryRep
is in mocking queries. If you need it otherwise, you can import it directly fromDatabase.Persist.Monad.SqlQueryRep
.
0.1.0.1
Fix quickstart
0.1.0.0
Initial release
SqlQueryT
+MonadSqlQuery
- Autogenerated persistent API
MockQueryT