Database.Esqueleto

The esqueleto EDSL (embedded domain specific language). This module replaces Database.Persist, so instead of importing that module you should just import this one:

 -- For a module using just esqueleto.
 import Database.Esqueleto

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 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.

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. 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.

However, it is not a goal to be able to write portable SQL. We do not try to hide the differences between DBMSs from you, and esqueleto code that works for one database may not work on another. This is a compromise we have to make in order to give you as much control over the raw SQL as possible without losing too much convenience. This also means that you may type-check a query that doesn't work on your DBMS.

Getting started

esqueleto's Language

class (Functor query, Applicative query, Monad query) => Esqueleto query expr backend | query -> expr backend, expr -> query backend where

Finally tagless representation of esqueleto's EDSL.

Methods

where_ :: expr (Value Bool) -> query ()

WHERE clause: restrict the query's result.

on :: expr (Value Bool) -> query ()

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 JOINs without ON clauses (either because you didn't do any JOIN, or because all JOINs already have their own ON clauses), a runtime exception OnClauseWithoutMatchingJoinException is thrown. ON clauses are optional when doing JOINs.

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 ons are applied on the reverse order that the JOINs 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.

groupBy :: ToSomeValues expr a => a -> query ()

GROUP BY clause. You can enclose multiple columns in a tuple.

 select $ from \(foo `InnerJoin` bar) -> do
   on (foo ^. FooBarId ==. bar ^. BarId)
   groupBy (bar ^. BarId, bar ^. BarName)
   return (bar ^. BarId, bar ^. BarName, countRows)

With groupBy you can sort by aggregate functions, like so (we used let to restrict the more general countRows to SqlExpr (Value Int) to avoid ambiguity):

 r <- select $ from \(foo `InnerJoin` bar) -> do
   on (foo ^. FooBarId ==. bar ^. BarId)
   groupBy $ bar ^. BarName
   let countRows' = countRows
   orderBy [asc countRows']
   return (bar ^. BarName, countRows')
 forM_ r $ \((Value name), (Value count)) -> do
   print name
   print (count :: Int)

orderBy :: [expr OrderBy] -> query ()

ORDER BY clause. See also asc and desc.

asc :: PersistField a => expr (Value a) -> expr OrderBy

Ascending order of this field or expression.

desc :: PersistField a => expr (Value a) -> expr OrderBy

Descending order of this field or expression.

limit :: Int64 -> query ()

LIMIT. Limit the number of returned rows.

offset :: Int64 -> query ()

OFFSET. Usually used with limit.

rand :: expr OrderBy

ORDER BY random() clause.

Since: 1.3.10

having :: expr (Value Bool) -> query ()

HAVING.

Since: 1.2.2

sub_select :: PersistField a => query (expr (Value a)) -> expr (Value a)

Execute a subquery SELECT in an expression. Returns a simple value so should be used only when the SELECT query is guaranteed to return just one row.

sub_selectDistinct :: PersistField a => query (expr (Value a)) -> expr (Value a)

Same as sub_select but using SELECT DISTINCT.

(^.) :: (PersistEntity val, PersistField typ) => expr (Entity val) -> EntityField val typ -> expr (Value typ)

Project a field of an entity.

(?.) :: (PersistEntity val, PersistField typ) => expr (Maybe (Entity val)) -> EntityField val typ -> expr (Value (Maybe typ))

Project a field of an entity that may be null.

val :: PersistField typ => typ -> expr (Value typ)

Lift a constant value from Haskell-land to the query.

isNothing :: PersistField typ => expr (Value (Maybe typ)) -> expr (Value Bool)

IS NULL comparison.

just :: expr (Value typ) -> expr (Value (Maybe typ))

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))

NULL value.

joinV :: expr (Value (Maybe (Maybe typ))) -> expr (Value (Maybe typ))

Join nested Maybes in a Value into one. This is useful when calling aggregate functions on nullable fields.

countRows :: Num a => expr (Value a)

COUNT(*) value.

count :: Num a => expr (Value typ) -> expr (Value a)

COUNT.

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)

random_ :: (PersistField a, Num a) => expr (Value a)

round_ :: (PersistField a, Num a, PersistField b, Num b) => expr (Value a) -> expr (Value b)

ceiling_ :: (PersistField a, Num a, PersistField b, Num b) => expr (Value a) -> expr (Value b)

floor_ :: (PersistField a, Num a, PersistField b, Num b) => expr (Value a) -> expr (Value b)

sum_ :: (PersistField a, PersistField b) => expr (Value a) -> expr (Value (Maybe b))

min_ :: PersistField a => expr (Value a) -> expr (Value (Maybe a))

max_ :: PersistField a => expr (Value a) -> expr (Value (Maybe a))

avg_ :: (PersistField a, PersistField b) => expr (Value a) -> expr (Value (Maybe b))

coalesce :: PersistField a => [expr (Value (Maybe a))] -> expr (Value (Maybe a))

COALESCE function. Evaluates the arguments in order and returns the value of the first non-NULL expression, or NULL (Nothing) otherwise. Some RDBMSs (such as SQLite) require at least two arguments; please refer to the appropriate documentation.

Since: 1.4.3

coalesceDefault :: PersistField a => [expr (Value (Maybe a))] -> expr (Value a) -> expr (Value a)

Like coalesce, but takes a non-nullable expression placed at the end of the expression list, which guarantees a non-NULL result.

Since: 1.4.3

like :: (PersistField s, IsString s) => expr (Value s) -> expr (Value s) -> expr (Value Bool)

LIKE operator.

(%) :: (PersistField s, IsString s) => expr (Value s)

The string %. May be useful while using like and concatenation (concat_ or ++., depending on your database). Note that you always to type the parenthesis, for example:

 name ``'like'`` (%) ++. val John ++. (%)

concat_ :: (PersistField s, IsString s) => [expr (Value s)] -> expr (Value s)

The CONCAT function with a variable number of parameters. Supported by MySQL and PostgreSQL.

(++.) :: (PersistField s, IsString s) => expr (Value s) -> expr (Value s) -> expr (Value s)

The || string concatenation operator (named after Haskell's ++ in order to avoid naming clash with ||.). Supported by SQLite and PostgreSQL.

subList_select :: PersistField a => query (expr (Value a)) -> expr (ValueList a)

Execute a subquery SELECT in an expression. Returns a list of values.

subList_selectDistinct :: PersistField a => query (expr (Value a)) -> expr (ValueList a)

Same as sublist_select but using SELECT DISTINCT.

valList :: PersistField typ => [typ] -> expr (ValueList typ)

Lift a list of constant value from Haskell-land to the query.

in_ :: PersistField typ => expr (Value typ) -> expr (ValueList typ) -> expr (Value Bool)

IN operator.

notIn :: PersistField typ => expr (Value typ) -> expr (ValueList typ) -> expr (Value Bool)

NOT IN operator.

exists :: query () -> expr (Value Bool)

EXISTS operator. For example:

 select $
 from $ \person -> do
 where_ $ exists $
          from $ \post -> do
          where_ (post ^. BlogPostAuthorId ==. person ^. PersonId)
 return person

notExists :: query () -> expr (Value Bool)

NOT EXISTS operator.

set :: PersistEntity val => expr (Entity val) -> [expr (Update val)] -> query ()

SET clause used on UPDATEs. 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)

(+=.) :: (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)

(<#) :: (a -> b) -> expr (Value a) -> expr (Insertion b)

Apply a PersistField constructor to expr Value arguments.

(<&>) :: expr (Insertion (a -> b)) -> expr (Value a) -> expr (Insertion b)

Apply extra expr Value arguments to a PersistField constructor

Instances

Esqueleto SqlQuery SqlExpr SqlBackend

from :: From query expr backend a => (a -> query b) -> query b

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 be NULL into scope. Used for OUTER JOINs.
• A JOIN of any other two types allowed by the innermost magic, where a JOIN may be an InnerJoin, a CrossJoin, a LeftOuterJoin, a RightOuterJoin, or a FullOuterJoin. The JOINs 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 JOINs. 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 RightOuterJoins or FullOuterJoins.

data Value a

A single value (as opposed to a whole entity). You may use (^.) or (?.) to get a Value from an Entity.

Constructors

Value a

Instances

Functor Value

Since: 1.4.4

Typeable1 Value

ToSomeValues SqlExpr (SqlExpr (Value a))

Eq a => Eq (Value a)

Ord a => Ord (Value a)

Show a => Show (Value a)

PersistField a => SqlSelect (SqlExpr (Value a)) (Value a)

You may return any single value (i.e. a single column) from a select query.

unValue :: Value a -> a

Unwrap a Value.

Since: 1.4.1

data ValueList a

A list of single values. There's a limited set of functions able to work with this data type (such as subList_select, valList, in_ and exists).

Constructors

ValueList a

Instances

Typeable1 ValueList

Eq a => Eq (ValueList a)

Ord a => Ord (ValueList a)

Show a => Show (ValueList a)

data OrderBy

Phantom type used by orderBy, asc and desc.

Joins

data InnerJoin a b

Data type that represents an INNER JOIN (see LeftOuterJoin for an example).

Constructors

a InnerJoin b

Instances

IsJoinKind InnerJoin

(Esqueleto query expr backend, FromPreprocess query expr backend (InnerJoin a b)) => From query expr backend (InnerJoin a b)

data CrossJoin a b

Data type that represents a CROSS JOIN (see LeftOuterJoin for an example).

Constructors

a CrossJoin b

Instances

IsJoinKind CrossJoin

(Esqueleto query expr backend, FromPreprocess query expr backend (CrossJoin a b)) => From query expr backend (CrossJoin a b)

data LeftOuterJoin a b

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.

Constructors

a LeftOuterJoin b

Instances

IsJoinKind LeftOuterJoin

(Esqueleto query expr backend, FromPreprocess query expr backend (LeftOuterJoin a b)) => From query expr backend (LeftOuterJoin a b)

data RightOuterJoin a b

Data type that represents a RIGHT OUTER JOIN (see LeftOuterJoin for an example).

Constructors

a RightOuterJoin b

Instances

IsJoinKind RightOuterJoin

(Esqueleto query expr backend, FromPreprocess query expr backend (RightOuterJoin a b)) => From query expr backend (RightOuterJoin a b)

data FullOuterJoin a b

Data type that represents a FULL OUTER JOIN (see LeftOuterJoin for an example).

Constructors

a FullOuterJoin b

Instances

IsJoinKind FullOuterJoin

(Esqueleto query expr backend, FromPreprocess query expr backend (FullOuterJoin a b)) => From query expr backend (FullOuterJoin a b)

data OnClauseWithoutMatchingJoinException

Exception thrown whenever on is used to create an ON clause but no matching JOIN is found.

Constructors

OnClauseWithoutMatchingJoinException String

Instances

Eq OnClauseWithoutMatchingJoinException

Ord OnClauseWithoutMatchingJoinException

Show OnClauseWithoutMatchingJoinException

Typeable OnClauseWithoutMatchingJoinException

Exception OnClauseWithoutMatchingJoinException

SQL backend

data SqlQuery a

SQL backend for esqueleto using SqlPersistT.

Instances

Monad SqlQuery

Functor SqlQuery

Applicative SqlQuery

Esqueleto SqlQuery SqlExpr SqlBackend

data SqlExpr a

An expression on the SQL backend.

Instances

Esqueleto SqlQuery SqlExpr SqlBackend

ToSomeValues SqlExpr (SqlExpr (Value a))

~ * a (Value b) => UnsafeSqlFunctionArgument (SqlExpr a)

PersistEntity a => SqlSelect (SqlExpr (Maybe (Entity a))) (Maybe (Entity a))

You may return a possibly-NULL Entity from a select query.

PersistEntity a => SqlSelect (SqlExpr (Entity a)) (Entity a)

You may return an Entity from a select query.

PersistField a => SqlSelect (SqlExpr (Value a)) (Value a)

You may return any single value (i.e. a single column) from a select query.

type SqlEntity ent = (PersistEntity ent, PersistEntityBackend ent ~ SqlBackend)

Constraint synonym for persistent entities whose backend is SqlPersistT.

select :: (SqlSelect a r, MonadIO m) => SqlQuery a -> SqlPersistT m [r]

Execute an esqueleto SELECT query inside persistent's SqlPersistT 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 (Entity v) for an entity v (i.e., like the * in SQL), which is then returned to Haskell-land as just Entity v.
• You may return a SqlExpr (Maybe (Entity v)) for an entity v that may be NULL, which is then returned to Haskell-land as Maybe (Entity v). Used for OUTER JOINs.
• You may return a SqlExpr (Value t) for a value t (i.e., a single column), where t is any instance of PersistField, which is then returned to Haskell-land as Value t. You may use Value to return projections of an Entity (see (^.) and (?.)) or to return any other value calculated on the query (e.g., countRows or sub_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, MonadIO m) => SqlQuery a -> SqlPersistT m [r]

Execute an esqueleto SELECT DISTINCT query inside persistent's SqlPersistT monad and return a list of rows.

selectSource :: (SqlSelect a r, MonadResource m) => SqlQuery a -> Source (SqlPersistT m) r

Execute an esqueleto SELECT query inside persistent's SqlPersistT monad and return a Source of rows.

selectDistinctSource :: (SqlSelect a r, MonadResource m) => SqlQuery a -> Source (SqlPersistT m) r

Execute an esqueleto SELECT DISTINCT query inside persistent's SqlPersistT monad and return a Source of rows.

delete :: MonadIO m => SqlQuery () -> SqlPersistT m ()

Execute an esqueleto DELETE query inside persistent's SqlPersistT 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 ()

deleteCount :: MonadIO m => SqlQuery () -> SqlPersistT m Int64

Same as delete, but returns the number of rows affected.

update :: (MonadIO m, SqlEntity val) => (SqlExpr (Entity val) -> SqlQuery ()) -> SqlPersistT m ()

Execute an esqueleto UPDATE query inside persistent's SqlPersistT 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_ $ isNothing (p ^. PersonAge)

updateCount :: (MonadIO m, SqlEntity val) => (SqlExpr (Entity val) -> SqlQuery ()) -> SqlPersistT m Int64

Same as update, but returns the number of rows affected.

insertSelect :: (MonadIO m, PersistEntity a) => SqlQuery (SqlExpr (Insertion a)) -> SqlPersistT m ()

Insert a PersistField for every selected value.

insertSelectDistinct :: (MonadIO m, PersistEntity a) => SqlQuery (SqlExpr (Insertion a)) -> SqlPersistT m ()

Insert a PersistField for every unique selected value.

Helpers

valkey :: (Esqueleto query expr backend, ToBackendKey SqlBackend entity, PersistField (Key entity)) => Int64 -> expr (Value (Key entity))

valkey i = val (Key (PersistInt64 i)) (https://github.com/meteficha/esqueleto/issues/9).

valJ :: (Esqueleto query expr backend, PersistField (Key entity)) => Value (Key entity) -> expr (Value (Key entity))

valJ is like val but for something that is already a Value. The use case it was written for was, given a Value lift the Key for that Value into the query expression in a type safe way. However, the implementation is more generic than that so we call it valJ.

Its important to note that the input entity and the output entity are constrained to be the same by the type signature on the function (https://github.com/prowdsponsor/esqueleto/pull/69).

Since: 1.4.2

Re-exports

We re-export many symbols from persistent for convenince:

• "Store functions" from Database.Persist.
• Everything from Database.Persist.Class except for PersistQuery and delete (use deleteKey instead).
• Everything from Database.Persist.Types except for Update, SelectOpt, BackendSpecificFilter and Filter.
• Everything from Database.Persist.Sql except for deleteWhereCount and updateWhereCount.

deleteKey :: (PersistStore (PersistEntityBackend val), MonadIO m, PersistEntity val) => Key val -> ReaderT (PersistEntityBackend val) m ()

Synonym for delete that does not clash with esqueleto's delete.

module Database.Persist.Sql