Safe Haskell | Safe-Infered |
---|
The esqueleto
EDSL (embedded domain specific language).
This module replaces Database.Persist
, so instead of
importing that module you should just import this one:
import Database.Esqueleto import qualified Database.Persist.Query as OldQuery
- class (Functor query, Applicative query, Monad query) => Esqueleto query expr backend | query -> expr backend, expr -> query backend where
- where_ :: expr (Value Bool) -> query ()
- on :: expr (Value Bool) -> query ()
- orderBy :: [expr OrderBy] -> query ()
- asc :: PersistField a => expr (Value a) -> expr OrderBy
- desc :: PersistField a => expr (Value a) -> expr OrderBy
- sub_select :: PersistField a => query (expr (Value a)) -> expr (Value a)
- sub_selectDistinct :: PersistField a => query (expr (Value a)) -> expr (Value a)
- (^.) :: (PersistEntity val, PersistField typ) => expr (Entity val) -> EntityField val typ -> expr (Value typ)
- (?.) :: (PersistEntity val, PersistField typ) => expr (Maybe (Entity val)) -> EntityField val typ -> expr (Value (Maybe typ))
- val :: PersistField typ => typ -> expr (Value typ)
- isNothing :: PersistField typ => expr (Value (Maybe typ)) -> expr (Value Bool)
- just :: expr (Value typ) -> expr (Value (Maybe typ))
- nothing :: expr (Value (Maybe typ))
- countRows :: Num a => expr (Value a)
- not_ :: expr (Value Bool) -> expr (Value Bool)
- (==.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)
- (>=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)
- (>.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)
- (<=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)
- (<.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)
- (!=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)
- (&&.) :: expr (Value Bool) -> expr (Value Bool) -> expr (Value Bool)
- (||.) :: expr (Value Bool) -> expr (Value Bool) -> expr (Value Bool)
- (+.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)
- (-.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)
- (/.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)
- (*.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)
- set :: PersistEntity val => expr (Entity val) -> [expr (Update val)] -> query ()
- (=.) :: (PersistEntity val, PersistField typ) => EntityField val typ -> expr (Value typ) -> expr (Update val)
- (+=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)
- (-=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)
- (*=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)
- (/=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)
- from :: From query expr backend a => (a -> query b) -> query b
- data Value a = Value a
- data OrderBy
- data InnerJoin a b = a InnerJoin b
- data CrossJoin a b = a CrossJoin b
- data LeftOuterJoin a b = a LeftOuterJoin b
- data RightOuterJoin a b = a RightOuterJoin b
- data FullOuterJoin a b = a FullOuterJoin b
- data OnClauseWithoutMatchingJoinException = OnClauseWithoutMatchingJoinException String
- data SqlQuery a
- data SqlExpr a
- select :: (SqlSelect a r, MonadLogger m, MonadResourceBase m) => SqlQuery a -> SqlPersist m [r]
- selectDistinct :: (SqlSelect a r, MonadLogger m, MonadResourceBase m) => SqlQuery a -> SqlPersist m [r]
- selectSource :: (SqlSelect a r, MonadLogger m, MonadResourceBase m) => SqlQuery a -> SqlPersist m (Source (ResourceT (SqlPersist m)) r)
- selectDistinctSource :: (SqlSelect a r, MonadLogger m, MonadResourceBase m) => SqlQuery a -> SqlPersist m (Source (ResourceT (SqlPersist m)) r)
- delete :: (MonadLogger m, MonadResourceBase m) => SqlQuery () -> SqlPersist m ()
- update :: (MonadLogger m, MonadResourceBase m, PersistEntity val, PersistEntityBackend val ~ SqlPersist) => (SqlExpr (Entity val) -> SqlQuery ()) -> SqlPersist m ()
- deleteKey :: (PersistStore backend m, PersistEntity val) => Key backend val -> backend m ()
- module Database.Persist.GenericSql
- module Database.Persist.Store
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
(http://www.yesodweb.com/book/persistent) to learn how to
define your schema.
Introduction
The main goals of esqueleto
are to:
- Be easily translatable to SQL. When you take a look at a
esqueleto
query, you should be able to know exactly how the SQL query will end up. (As opposed to being a relational algebra EDSL such as HaskellDB, which is non-trivial to translate into SQL.) - Support the mostly used SQL features. We'd like you to be
able to use
esqueleto
for all of your queries, no exceptions. Send a pull request or open an issue on our project page (https://github.com/meteficha/esqueleto) if there's anything missing that you'd like to see. - Be as type-safe as possible. There are ways of shooting
yourself in the foot while using
esqueleto
because it's extremely hard to provide 100% type-safety into a SQL-like EDSL---there's a tension between supporting features with a nice syntax and rejecting bad code. However, we strive to provide as many type checks as possible. If you get bitten by some invalid code that type-checks, please open an issue on our project page so we can take a look.
Getting started
We like clean, easy-to-read EDSLs. However, in order to achieve this goal we've used a lot of type hackery, leading to some hard-to-read type signatures. On this section, we'll try to build some intuition about the syntax.
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 |]
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:
SELECT * FROM Person
In esqueleto
, we may write the same query above as:
do people <-select
$from
$ \person -> do return person liftIO $ mapM_ (putStrLn . personName . entityVal) people
The expression above has type SqlPersist m ()
, while
people
has type [Entity Person]
. The query above will be
translated into exactly the same query we wrote manually, but
instead of SELECT *
it will list all entity fields (using
*
is not robust). Note that esqueleto
knows that we want
an Entity Person
just because of the personName
that we're
printing later.
However, most of the time we need to filter our queries using
WHERE
. For example:
SELECT * FROM Person WHERE Person.name = "John"
In esqueleto
, we may write the same query above as:
select $ from $ \p -> dowhere_
(p^.
PersonName==.
val
"John") return p
Although esqueleto
's code is a bit more noisy, it's has
almost the same structure (save from the return
). 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 would be:
SELECT * FROM Person WHERE Person.age >= 18
In esqueleto
, we may write the same query above as:
select $ from $ \p -> do where_ (p ^. PersonAge>=.
just
(val 18)) return p
Since age
is an optional Person
field, we use just
lift
val 18 :: SqlExpr (Value Int)
into just (val 18) ::
SqlExpr (Value (Just Int))
.
Implicit joins are represented by tuples. For example, to get the list of all blog posts and their authors, we could write:
SELECT BlogPost.*, Person.* FROM BlogPost, Person WHERE BlogPost.authorId = Person.id ORDER BY BlogPost.title ASC
In esqueleto
, we may write the same query above as:
select $ from $ \(b, p) -> do where_ (b ^. BlogPostAuthorId ==. p ^. PersonId)orderBy
[asc
(b ^. BlogPostTitle)] return (b, p)
However, we may want your results to include people who don't
have any blog posts as well using a LEFT OUTER JOIN
:
SELECT Person.*, BlogPost.* FROM Person LEFT OUTER JOIN BlogPost ON Person.id = BlogPost.authorId ORDER BY Person.name ASC, BlogPost.title ASC
In esqueleto
, we may write the same query above as:
select $ from $ \(p `LeftOuterJoin
` mb) -> doon
(just (p ^. PersonId) ==. mb?.
BlogPostAuthorId) orderBy [asc (p ^. PersonName), asc (mb?.
BlogPostTitle)] return (p, mb)
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
the Follow
entity:
SELECT P1.*, Follow.*, P2.* FROM Person AS P1 INNER JOIN Follow ON P1.id = Follow.follower INNER JOIN P2 ON P2.id = Follow.followed
In esqueleto
, we may write the same query above as:
select $ from $ \(p1 `InnerJoin
` f `InnerJoin
` p2) -> do on (p2 ^. PersonId ==. f ^. FollowFollowed) on (p1 ^. PersonId ==. f ^. FollowFollower) return (p1, f, p2)
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 documention of
on
for more details).
We also currently supports UPDATE
and DELETE
statements.
For example:
doupdate
$ \p -> doset
p [ PersonName=.
val "João" ] where_ (p ^. PersonName ==. val "Joao")delete
$ from $ \p -> do where_ (p ^. PersonAge <. just (val 14))
esqueleto
's Language
class (Functor query, Applicative query, Monad query) => Esqueleto query expr backend | query -> expr backend, expr -> query backend whereSource
Finally tagless representation of esqueleto
's EDSL.
where_ :: expr (Value Bool) -> query ()Source
WHERE
clause: restrict the query's result.
on :: expr (Value Bool) -> query ()Source
ON
clause: restrict the a JOIN
's result. The ON
clause will be applied to the last JOIN
that does not
have an ON
clause yet. If there are no JOIN
s without
ON
clauses (either because you didn't do any JOIN
, or
because all JOIN
s already have their own ON
clauses), a
runtime exception OnClauseWithoutMatchingJoinException
is
thrown. ON
clauses are optional when doing JOIN
s.
On the simple case of doing just one JOIN
, for example
select $
from $ \(foo `InnerJoin
` bar) -> do
on (foo ^. FooId ==. bar ^. BarFooId)
...
there's no ambiguity and the rules above just mean that
you're allowed to call on
only once (as in SQL). If you
have many joins, then the on
s are applied on the reverse
order that the JOIN
s appear. For example:
select $ from $ \(foo `InnerJoin
` bar `InnerJoin
` baz) -> do on (baz ^. BazId ==. bar ^. BarBazId) on (foo ^. FooId ==. bar ^. BarFooId) ...
The order is reversed in order to improve composability.
For example, consider query1
and query2
below:
let query1 = from $ \(foo `InnerJoin
` bar) -> do on (foo ^. FooId ==. bar ^. BarFooId) query2 = from $ \(mbaz `LeftOuterJoin
` quux) -> do return (mbaz ?. BazName, quux) test1 = (,) <$> query1 <*> query2 test2 = flip (,) <$> query2 <*> query1
If the order was not reversed, then test2
would be
broken: query1
's on
would refer to query2
's
LeftOuterJoin
.
orderBy :: [expr OrderBy] -> query ()Source
asc :: PersistField a => expr (Value a) -> expr OrderBySource
Ascending order of this field or expression.
desc :: PersistField a => expr (Value a) -> expr OrderBySource
Descending order of this field or expression.
sub_select :: PersistField a => query (expr (Value a)) -> expr (Value a)Source
Execute a subquery SELECT
in an expression.
sub_selectDistinct :: PersistField a => query (expr (Value a)) -> expr (Value a)Source
Execute a subquery SELECT_DISTINCT
in an expression.
(^.) :: (PersistEntity val, PersistField typ) => expr (Entity val) -> EntityField val typ -> expr (Value typ)Source
Project a field of an entity.
(?.) :: (PersistEntity val, PersistField typ) => expr (Maybe (Entity val)) -> EntityField val typ -> expr (Value (Maybe typ))Source
Project a field of an entity that may be null.
val :: PersistField typ => typ -> expr (Value typ)Source
Lift a constant value from Haskell-land to the query.
isNothing :: PersistField typ => expr (Value (Maybe typ)) -> expr (Value Bool)Source
IS NULL
comparison.
just :: expr (Value typ) -> expr (Value (Maybe typ))Source
Analogous to Just
, promotes a value of type typ
into
one of type Maybe typ
. It should hold that val . Just
=== just . val
.
nothing :: expr (Value (Maybe typ))Source
NULL
value.
countRows :: Num a => expr (Value a)Source
COUNT(*)
value.
not_ :: expr (Value Bool) -> expr (Value Bool)Source
(==.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)Source
(>=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)Source
(>.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)Source
(<=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)Source
(<.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)Source
(!=.) :: PersistField typ => expr (Value typ) -> expr (Value typ) -> expr (Value Bool)Source
(&&.) :: expr (Value Bool) -> expr (Value Bool) -> expr (Value Bool)Source
(||.) :: expr (Value Bool) -> expr (Value Bool) -> expr (Value Bool)Source
(+.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)Source
(-.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)Source
(/.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)Source
(*.) :: PersistField a => expr (Value a) -> expr (Value a) -> expr (Value a)Source
set :: PersistEntity val => expr (Entity val) -> [expr (Update val)] -> query ()Source
SET
clause used on UPDATE
s. Note that while it's not
a type error to use this function on a SELECT
, it will
most certainly result in a runtime error.
(=.) :: (PersistEntity val, PersistField typ) => EntityField val typ -> expr (Value typ) -> expr (Update val)Source
(+=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)Source
(-=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)Source
(*=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)Source
(/=.) :: (PersistEntity val, PersistField a) => EntityField val a -> expr (Value a) -> expr (Update val)Source
from :: From query expr backend a => (a -> query b) -> query bSource
FROM
clause: bring entities into scope.
This function internally uses two type classes in order to provide some flexibility of how you may call it. Internally we refer to these type classes as the two different magics.
The innermost magic allows you to use from
with the
following types:
-
expr (Entity val)
, which brings a single entity into scope. -
expr (Maybe (Entity val))
, which brings a single entity that may beNULL
into scope. Used forOUTER JOIN
s. - A
JOIN
of any other two types allowed by the innermost magic, where aJOIN
may be anInnerJoin
, aCrossJoin
, aLeftOuterJoin
, aRightOuterJoin
, or aFullOuterJoin
. TheJOINs
have right fixity, the same as in SQL.
The outermost magic allows you to use from
on any tuples of
types supported by innermost magic (and also tuples of tuples,
and so on), up to 8-tuples.
Note that using from
for the same entity twice does work and
corresponds to a self-join. You don't even need to use two
different calls to from
, you may use a JOIN
or a tuple.
The following are valid examples of uses of from
(the types
of the arguments of the lambda are inside square brackets):
from $ \person -> ... from $ \(person, blogPost) -> ... from $ \(p `LeftOuterJoin
` mb) -> ... from $ \(p1 `InnerJoin
` f `InnerJoin
` p2) -> ... from $ \((p1 `InnerJoin
` f) `InnerJoin
` p2) -> ...
The types of the arguments to the lambdas above are, respectively:
person :: ( Esqueleto query expr backend , PersistEntity Person , PersistEntityBackend Person ~ backend ) => expr (Entity Person) (person, blogPost) :: (...) => (expr (Entity Person), expr (Entity BlogPost)) (p `LeftOuterJoin
` mb) :: (...) => InnerJoin (expr (Entity Person)) (expr (Maybe (Entity BlogPost))) (p1 `InnerJoin
` f `InnerJoin
` p2) :: (...) => InnerJoin (expr (Entity Person)) (InnerJoin (expr (Entity Follow)) (expr (Entity Person))) ((p1 `InnerJoin
` f) `InnerJoin
` p2) :: :: (...) => InnerJoin (InnerJoin (expr (Entity Person)) (expr (Entity Follow))) (expr (Entity Person))
Note that some backends may not support all kinds of JOIN
s.
For example, when using the SQL backend with SQLite, it will
not accept the last example above (which is associated to the
left, instead of being to the right) and will not accept
RightOuterJoin
s or FullOuterJoin
s.
A single value (as opposed to a whole entity). You may use
(
or ^.
)(
to get a ?.
)Value
from an Entity
.
Value a |
Joins
Data type that represents an INNER JOIN
(see LeftOuterJoin
for an example).
a InnerJoin b |
IsJoinKind InnerJoin | |
(Esqueleto query expr backend, FromPreprocess query expr backend (InnerJoin a b)) => From query expr backend (InnerJoin a b) |
Data type that represents a CROSS JOIN
(see LeftOuterJoin
for an example).
a CrossJoin b |
IsJoinKind CrossJoin | |
(Esqueleto query expr backend, FromPreprocess query expr backend (CrossJoin a b)) => From query expr backend (CrossJoin a b) |
data LeftOuterJoin a b Source
Data type that represents a LEFT OUTER JOIN
. For example,
select $
from $ \(person `LeftOuterJoin
` pet) ->
...
is translated into
SELECT ... FROM Person LEFT OUTER JOIN Pet ...
See also: from
.
a LeftOuterJoin b |
IsJoinKind LeftOuterJoin | |
(Esqueleto query expr backend, FromPreprocess query expr backend (LeftOuterJoin a b)) => From query expr backend (LeftOuterJoin a b) |
data RightOuterJoin a b Source
Data type that represents a RIGHT OUTER JOIN
(see LeftOuterJoin
for an example).
a RightOuterJoin b |
IsJoinKind RightOuterJoin | |
(Esqueleto query expr backend, FromPreprocess query expr backend (RightOuterJoin a b)) => From query expr backend (RightOuterJoin a b) |
data FullOuterJoin a b Source
Data type that represents a FULL OUTER JOIN
(see LeftOuterJoin
for an example).
a FullOuterJoin b |
IsJoinKind FullOuterJoin | |
(Esqueleto query expr backend, FromPreprocess query expr backend (FullOuterJoin a b)) => From query expr backend (FullOuterJoin a b) |
data OnClauseWithoutMatchingJoinException Source
Exception thrown whenever on
is used to create an ON
clause but no matching JOIN
is found.
SQL backend
SQL backend for esqueleto
using SqlPersist
.
An expression on the SQL backend.
Esqueleto SqlQuery SqlExpr SqlPersist | |
PersistEntity a => SqlSelect (SqlExpr (Maybe (Entity a))) (Maybe (Entity a)) | |
PersistEntity a => SqlSelect (SqlExpr (Entity a)) (Entity a) | |
PersistField a => SqlSelect (SqlExpr (Value a)) (Value a) | You may return any single value (i.e. a single column) from
a |
select :: (SqlSelect a r, MonadLogger m, MonadResourceBase m) => SqlQuery a -> SqlPersist m [r]Source
Execute an esqueleto
SELECT
query inside persistent
's
SqlPersist
monad and return a list of rows.
We've seen that from
has some magic about which kinds of
things you may bring into scope. This select
function also
has some magic for which kinds of things you may bring back to
Haskell-land by using SqlQuery
's return
:
- You may return a
SqlExpr (
for an entityEntity
v)v
(i.e., like the*
in SQL), which is then returned to Haskell-land as justEntity v
. - You may return a
SqlExpr (Maybe (Entity v))
for an entityv
that may beNULL
, which is then returned to Haskell-land asMaybe (Entity v)
. Used forOUTER JOIN
s. - You may return a
SqlExpr (
for a valueValue
t)t
(i.e., a single column), wheret
is any instance ofPersistField
, which is then returned to Haskell-land asValue t
. You may useValue
to return projections of anEntity
(see(
and^.
)(
) or to return any other value calculated on the query (e.g.,?.
)countRows
orsub_select
).
The SqlSelect a r
class has functional dependencies that
allow type information to flow both from a
to r
and
vice-versa. This means that you'll almost never have to give
any type signatures for esqueleto
queries. For example, the
query select $ from $ \p -> return p
alone is ambiguous, but
in the context of
do ps <- select $ from $ \p -> return p liftIO $ mapM_ (putStrLn . personName . entityVal) ps
we are able to infer from that single personName . entityVal
function composition that the p
inside the query is of type
SqlExpr (Entity Person)
.
selectDistinct :: (SqlSelect a r, MonadLogger m, MonadResourceBase m) => SqlQuery a -> SqlPersist m [r]Source
Execute an esqueleto
SELECT DISTINCT
query inside
persistent
's SqlPersist
monad and return a list of rows.
selectSource :: (SqlSelect a r, MonadLogger m, MonadResourceBase m) => SqlQuery a -> SqlPersist m (Source (ResourceT (SqlPersist m)) r)Source
Execute an esqueleto
SELECT
query inside persistent
's
SqlPersist
monad and return a Source
of rows.
selectDistinctSource :: (SqlSelect a r, MonadLogger m, MonadResourceBase m) => SqlQuery a -> SqlPersist m (Source (ResourceT (SqlPersist m)) r)Source
Execute an esqueleto
SELECT DISTINCT
query inside
persistent
's SqlPersist
monad and return a Source
of
rows.
delete :: (MonadLogger m, MonadResourceBase m) => SqlQuery () -> SqlPersist m ()Source
Execute an esqueleto
DELETE
query inside persistent
's
SqlPersist
monad. Note that currently there are no type
checks for statements that should not appear on a DELETE
query.
Example of usage:
delete $ from $ \appointment -> where_ (appointment ^. AppointmentDate <. val now)
Unlike select
, there is a useful way of using delete
that
will lead to type ambiguities. If you want to delete all rows
(i.e., no where_
clause), you'll have to use a type signature:
delete $ from $ \(appointment :: SqlExpr (Entity Appointment)) -> return ()
update :: (MonadLogger m, MonadResourceBase m, PersistEntity val, PersistEntityBackend val ~ SqlPersist) => (SqlExpr (Entity val) -> SqlQuery ()) -> SqlPersist m ()Source
Execute an esqueleto
UPDATE
query inside persistent
's
SqlPersist
monad. Note that currently there are no type
checks for statements that should not appear on a UPDATE
query.
Example of usage:
update $ p -> do set p [ PersonAge =. just (val thisYear) -. p ^. PersonBorn ] where_ $ isNull (p ^. PersonAge)
Re-exports
We re-export Database.Persist.Store
for convenience, since
esqueleto
currently does not provide a way of doing
INSERT
s.
deleteKey :: (PersistStore backend m, PersistEntity val) => Key backend val -> backend m ()Source
module Database.Persist.GenericSql
module Database.Persist.Store