Esqueleto
Image courtesy Chrissy Long
Esqueleto, a SQL DSL for Haskell
Esqueleto is a bare bones, type-safe EDSL for SQL queries that works with unmodified persistent SQL backends. The name of this library means "skeleton" in Portuguese and contains all three SQL letters in the correct order =). It was inspired by Scala's Squeryl but created from scratch. Its language closely resembles SQL. Currently, SELECTs, UPDATEs, INSERTs and DELETEs are supported.
In particular, esqueleto is the recommended library for type-safe JOINs on persistent SQL backends. (The alternative is using raw SQL, but that's error prone and does not offer any composability.). For more information read esqueleto.
Setup
If you're already using persistent
, then you're ready to use esqueleto
, no further setup is needed. If you're just starting a new project and would like to use esqueleto
, take a look at persistent
's book first to learn how to define your schema.
If you need to use persistent
's default support for queries as well, either import it qualified:
-- For a module that mostly uses esqueleto.
import Database.Esqueleto
import qualified Database.Persistent as P
or import esqueleto
itself qualified:
-- For a module that uses esqueleto just on some queries.
import Database.Persistent
import qualified Database.Esqueleto as E
Other than identifier name clashes, esqueleto
does not conflict with persistent
in any way.
Goals
The main goals of esqueleto
are:
- Be easily translatable to SQL. (You should be able to know exactly how the SQL query will end up.)
- Support the most widely used SQL features.
- Be as type-safe as possible.
It is not a goal to be able to write portable SQL. We do not try to hide the differences between DBMSs from you
Introduction
For the following examples, we'll use this example schema:
share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persist|
Person
name String
age Int Maybe
deriving Eq Show
BlogPost
title String
authorId PersonId
deriving Eq Show
Follow
follower PersonId
followed PersonId
deriving Eq Show
|]
Select
Most of esqueleto
was created with SELECT
statements in mind, not only because they're the most common but also because they're the most complex kind of statement. The most simple kind of SELECT
would be:
putPersons :: SqlPersist m ()
putPersons = do
people <- select $
from $ \person -> do
return person
liftIO $ mapM_ (putStrLn . personName . entityVal) people
which generates this SQL:
SELECT *
FROM Person
esqueleto
knows that we want an Entity Person
just because of the personName
that is printed.
Where
Filtering by PersonName
:
select $
from $ \p -> do
where_ (p ^. PersonName ==. val "John")
return p
which generates this SQL:
SELECT *
FROM Person
WHERE Person.name = "John"
The (^.)
operator is used to project a field from an entity. The field name is the same one generated by persistent
s Template Haskell functions. We use val
to lift a constant Haskell value into the SQL query.
Another example:
In esqueleto
, we may write the same query above as:
select $
from $ \p -> do
where_ (p ^. PersonAge >=. just (val 18))
return p
which generates this SQL:
SELECT *
FROM Person
WHERE Person.age >= 18
Since age
is an optional Person
field, we use just
to liftval 18 :: SqlExpr (Value Int)
into just (val 18) ::SqlExpr (Value (Maybe Int))
.
Joins
Implicit joins are represented by tuples.
For example, to get the list of all blog posts and their authors, we could write:
select $
from $ \(b, p) -> do
where_ (b ^. BlogPostAuthorId ==. p ^. PersonId)
orderBy [asc (b ^. BlogPostTitle)]
return (b, p)
which generates this SQL:
SELECT BlogPost.*, Person.*
FROM BlogPost, Person
WHERE BlogPost.authorId = Person.id
ORDER BY BlogPost.title ASC
However, you may want your results to include people who don't have any blog posts as well using a LEFT OUTER JOIN
:
select $
from $ \(p `LeftOuterJoin` mb) -> do
on (just (p ^. PersonId) ==. mb ?. BlogPostAuthorId)
orderBy [asc (p ^. PersonName), asc (mb ?. BlogPostTitle)]
return (p, mb)
which generates this SQL:
SELECT Person.*, BlogPost.*
FROM Person LEFT OUTER JOIN BlogPost
ON Person.id = BlogPost.authorId
ORDER BY Person.name ASC, BlogPost.title ASC
Left Outer Join
On a LEFT OUTER JOIN
the entity on the right hand side may not exist (i.e. there may be a Person
without any BlogPost
s), so while p :: SqlExpr (Entity Person)
, we have mb :: SqlExpr (Maybe (Entity BlogPost))
. The whole expression above has type SqlPersist m [(Entity Person, Maybe (Entity BlogPost))]
. Instead of using (^.)
, we used (?.)
to project a field from a Maybe (Entity a)
.
We are by no means limited to joins of two tables, nor by joins of different tables. For example, we may want a list of the Follow
entity:
select $
from $ \(p1 `InnerJoin` f `InnerJoin` p2) -> do
on (p2 ^. PersonId ==. f ^. FollowFollowed)
on (p1 ^. PersonId ==. f ^. FollowFollower)
return (p1, f, p2)
which generates this SQL:
SELECT P1.*, Follow.*, P2.*
FROM Person AS P1
INNER JOIN Follow ON P1.id = Follow.follower
INNER JOIN Person AS P2 ON P2.id = Follow.followed
Note carefully that the order of the ON clauses is reversed! You're required to write your on
s in reverse order because that helps composability (see the documentation of on
for more details).
Update and Delete
do update $ \p -> do
set p [ PersonName =. val "João" ]
where_ (p ^. PersonName ==. val "Joao")
delete $
from $ \p -> do
where_ (p ^. PersonAge <. just (val 14))
The results of queries can also be used for insertions. In SQL
, we might write the following, inserting a new blog post for every user:
insertSelect $ from $ \p->
return $ BlogPost <# "Group Blog Post" <&> (p ^. PersonId)
which generates this SQL:
INSERT INTO BlogPost
SELECT ('Group Blog Post', id)
FROM Person
Individual insertions can be performed through Persistent's insert
function, reexported for convenience.
Re-exports
We re-export many symbols from persistent
for convenience:
- "Store functions" from "Database.Persist".
- Everything from "Database.Persist.Class" except for
PersistQuery
anddelete
(usedeleteKey
instead). - Everything from "Database.Persist.Types" except for
Update
,SelectOpt
,BackendSpecificFilter
andFilter
. - Everything from "Database.Persist.Sql" except for
deleteWhereCount
andupdateWhereCount
.
RDBMS Specific
There are many differences between SQL syntax and functions supported by different RDBMSs. Since version 2.2.8, esqueleto
includes modules containing functions that are specific to a given RDBMS.
- PostgreSQL:
Database.Esqueleto.PostgreSQL
- MySQL:
Database.Esqueleto.MySQL
- SQLite:
Database.Esqueleto.SQLite
In order to use these functions, you need to explicitly import their corresponding modules.
Unsafe functions, operators and values
Esqueleto doesn't support every possible function, and it can't - many functions aren't available on every RDBMS platform, and sometimes the same functionality is hidden behind different names. To overcome this problem, Esqueleto exports a number of unsafe functions to call any function, operator or value. These functions can be found in Database.Esqueleto.Internal.Sql module.
Warning: the functions discussed in this section must always be used with an explicit type signature,and the user must be careful to provide a type signature that corresponds correctly with the underlying code. The functions have extremely general types, and if you allow type inference to figure everything out for you, it may not correspond with the underlying SQL types that you want. This interface is effectively the FFI to SQL database, so take care!
The most common use of these functions is for calling RDBMS specific or custom functions,
for that end we use unsafeSqlFunction
. For example, if we wish to consult the postgres
now
function we could so as follow:
postgresTime :: (MonadIO m, MonadLogger m) => SqlWriteT m UTCTime
postgresTime =
result <- select (pure now)
case result of
[x] -> pure x
_ -> error "now() is guaranteed to return a single result"
where
now :: SqlExpr (Value UTCTime)
now = unsafeSqlFunction "now" ()
which generates this SQL:
SELECT now()
With the now
function we could now use the current time of the postgres RDBMS on any query.
Do notice that now
does not use any arguments, so we use ()
that is an instance of
UnsafeSqlFunctionArgument
to represent no arguments, an empty list cast to a correct value
will yield the same result as ()
.
We can also use unsafeSqlFunction
for more complex functions with customs values using
unsafeSqlValue
which turns any string into a sql value of whatever type we want, disclaimer:
if you use it badly you will cause a runtime error. For example, say we want to try postgres'
date_part
function and get the day of a timestamp, we could use:
postgresTimestampDay :: (MonadIO m, MonadLogger m) => SqlWriteT m Int
postgresTimestampDay =
result <- select (return $ dayPart date)
case result of
[x] -> pure x
_ -> error "dayPart is guaranteed to return a single result"
where
dayPart :: SqlExpr (Value UTCTime) -> SqlExpr (Value Int)
dayPart s = unsafeSqlFunction "date_part" (unsafeSqlValue "\'day\'" :: SqlExpr (Value String) ,s)
date :: SqlExpr (Value UTCTime)
date = unsafeSqlValue "TIMESTAMP \'2001-02-16 20:38:40\'"
which generates this SQL:
SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40')
Using unsafeSqlValue
we were required to also define the type of the value.
Another useful unsafe function is unsafeSqlCastAs
, which allows us to cast any type
to another within a query. For example, say we want to use our previews dayPart
function
on the current system time, we could:
postgresTimestampDay :: (MonadIO m, MonadLogger m) => SqlWriteT m Int
postgresTimestampDay = do
currentTime <- liftIO getCurrentTime
result <- select (return $ dayPart (toTIMESTAMP $ val currentTime))
case result of
[x] -> pure x
_ -> error "dayPart is guaranteed to return a single result"
where
dayPart :: SqlExpr (Value UTCTime) -> SqlExpr (Value Int)
dayPart s = unsafeSqlFunction "date_part" (unsafeSqlValue "\'day\'" :: SqlExpr (Value String) ,s)
toTIMESTAMP :: SqlExpr (Value UTCTime) -> SqlExpr (Value UTCTime)
toTIMESTAMP = unsafeSqlCastAs "TIMESTAMP"
which generates this SQL:
SELECT date_part('day', CAST('2019-10-28 23:19:39.400898344Z' AS TIMESTAMP))
SQL injection
Esqueleto uses parameterization to prevent sql injections on values and arguments on all queries, for example, if we have:
myEvilQuery :: (MonadIO m, MonadLogger m) => SqlWriteT m ()
myEvilQuery =
select (return $ val ("hi\'; DROP TABLE foo; select \'bye\'" :: String)) >>= liftIO . print
which generates this SQL(when using postgres):
SELECT 'hi''; DROP TABLE foo; select ''bye'''
And the printed value is hi\'; DROP TABLE foo; select \'bye\'
and no table is dropped. This is good
and makes the use of strings values safe. Unfortunately this is not the case when using unsafe functions.
Let's see an example of defining a new evil now
function:
myEvilQuery :: (MonadIO m, MonadLogger m) => SqlWriteT m ()
myEvilQuery =
select (return nowWithInjection) >>= liftIO . print
where
nowWithInjection :: SqlExpr (Value UTCTime)
nowWithInjection = unsafeSqlFunction "0; DROP TABLE bar; select now" ([] :: [SqlExpr (Value Int)])
which generates this SQL:
SELECT 0; DROP TABLE bar; select now()
If we were to run the above code we would see the postgres time printed but the table bar
will be erased with no indication whatsoever. Another example of this behavior is seen when using
unsafeSqlValue
:
myEvilQuery :: (MonadIO m, MonadLogger m) => SqlWriteT m ()
myEvilQuery =
select (return $ dayPart dateWithInjection) >>= liftIO . print
where
dayPart :: SqlExpr (Value UTCTime) -> SqlExpr (Value Int)
dayPart s = unsafeSqlFunction "date_part" (unsafeSqlValue "\'day\'" :: SqlExpr (Value String) ,s)
dateWithInjection :: SqlExpr (Value UTCTime)
dateWithInjection = unsafeSqlValue "TIMESTAMP \'2001-02-16 20:38:40\');DROP TABLE bar; select (16"
which generates this SQL:
SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40');DROP TABLE bar; select (16)
This will print 16 and also erase the bar
table. The main take away of this examples is to
never use any user or third party input inside an unsafe function without first parsing it or
heavily sanitizing the input.
Tests and Postgres
To run the tests, do stack test
. This tests all the backends, so you'll need
to have MySQL and Postgresql installed.
Using apt-get, you should be able to do:
sudo apt-get install postgresql postgresql-contrib
sudo apt-get install libpq-dev
Using homebrew on OSx
brew install postgresql
brew install libpq
Detailed instructions on the Postgres wiki here
The connection details are located near the bottom of the test/PostgreSQL/Test.hs file:
withConn =
R.runResourceT . withPostgresqlConn "host=localhost port=5432 user=esqutest password=esqutest dbname=esqutest"
You can change these if you like but to just get them working set up as follows on linux:
$ sudo -u postgres createuser esqutest
$ sudo -u postgres createdb esqutest
$ sudo -u postgres psql
postgres=# \password esqutest
And on osx
$ createuser esqutest
$ createdb esqutest
$ psql postgres
postgres=# \password esqutest