WHERE clause: restrict the query's result.

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.

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 SqlSqlExpr (Value Int) to avoid ambiguity---the second use of countRows has its type restricted by the :: Int below):

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)

ORDER BY clause. See also asc and desc.

Multiple calls to orderBy get concatenated on the final query, including distinctOnOrderBy.

ORDER BY random() clause.

Since: 1.3.10

Ascending order of this field or SqlExpression.

Descending order of this field or SqlExpression.

LIMIT. Limit the number of returned rows.

OFFSET. Usually used with limit.

DISTINCT. Change the current SELECT into SELECT DISTINCT. For example:

select $ distinct $
  from \foo -> do
  ...

Note that this also has the same effect:

select $
  from \foo -> do
  distinct (return ())
  ...

Since: 2.2.4

DISTINCT ON. Change the current SELECT into SELECT DISTINCT ON (SqlExpressions). For example:

select $
  from \foo ->
  distinctOn [don (foo ^. FooName), don (foo ^. FooState)] $ do
  ...

You can also chain different calls to distinctOn. The above is equivalent to:

select $
  from \foo ->
  distinctOn [don (foo ^. FooName)] $
  distinctOn [don (foo ^. FooState)] $ do
  ...

Each call to distinctOn adds more SqlExpressions. Calls to distinctOn override any calls to distinct.

Note that PostgreSQL requires the SqlExpressions on DISTINCT ON to be the first ones to appear on a ORDER BY. This is not managed automatically by esqueleto, keeping its spirit of trying to be close to raw SQL.

Supported by PostgreSQL only.

Since: 2.2.4

Erase an SqlExpression's type so that it's suitable to be used by distinctOn.

Since: 2.2.4

A convenience function that calls both distinctOn and orderBy. In other words,

distinctOnOrderBy [asc foo, desc bar, desc quux] $ do
  ...

is the same as:

distinctOn [don foo, don  bar, don  quux] $ do
  orderBy  [asc foo, desc bar, desc quux]
  ...

Since: 2.2.4

HAVING.

Since: 1.2.2

Add a locking clause to the query. Please read LockingKind documentation and your RDBMS manual.

If multiple calls to locking are made on the same query, the last one is used.

Since: 2.2.7

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

Project a field of an entity.

Project a field of an entity that may be null.

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

IS NULL comparison.

Analogous to Just, promotes a value of type typ into one of type Maybe typ. It should hold that val . Just === just . val.

NULL value.

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

Project an SqlExpression that may be null, guarding against null cases.

COUNT(*) value.

COUNT.

COUNT(DISTINCT x).

Since: 2.4.1

Allow a number of one type to be used as one of another type via an implicit cast. An explicit cast is not made, this function changes only the types on the Haskell side.

Caveat: Trying to use castNum from Double to Int will not result in an integer, the original fractional number will still be used! Use round_, ceiling_ or floor_ instead.

Safety: This operation is mostly safe due to the Num constraint between the types and the fact that RDBMSs usually allow numbers of different types to be used interchangeably. However, there may still be issues with the query not being accepted by the RDBMS or persistent not being able to parse it.

Since: 2.2.9

Same as castNum, but for nullable values.

Since: 2.2.9

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

Since: 1.4.3

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

Since: 1.4.3

LOWER function.

LIKE operator.

ILIKE operator (case-insensitive LIKE).

Supported by PostgreSQL only.

Since: 2.2.3

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

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

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

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

Cast a string type into Text. This function is very useful if you want to use newtypes, or if you want to apply functions such as like to strings of different types.

Safety: This is a slightly unsafe function, especially if you have defined your own instances of SqlString. Also, since Maybe is an instance of SqlString, it's possible to turn a nullable value into a non-nullable one. Avoid using this function if possible.

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

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

Same as just but for ValueList. Most of the time you won't need it, though, because you can use just from inside subList_select or Just from inside valList.

Since: 2.2.12

IN operator. For example if you want to select all Persons by a list of IDs:

SELECT *
FROM Person
WHERE Person.id IN (?)

In esqueleto, we may write the same query above as:

select $
from $ \person -> do
where_ $ person ^. PersonId `in_` valList personIds
return person

Where personIds is of type [Key Person].

NOT IN operator.

EXISTS operator. For example:

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

NOT EXISTS operator.

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.

CASE statement. For example:

select $
return $
case_
   [ when_
       (exists $
       from $ \p -> do
       where_ (p ^. PersonName ==. val "Mike"))
     then_
       (sub_select $
       from $ \v -> do
       let sub =
               from $ \c -> do
               where_ (c ^. PersonName ==. val "Mike")
               return (c ^. PersonFavNum)
       where_ (v ^. PersonFavNum >. sub_select sub)
       return $ count (v ^. PersonName) +. val (1 :: Int)) ]
   (else_ $ val (-1))

This query is a bit complicated, but basically it checks if a person named "Mike" exists, and if that person does, run the subquery to find out how many people have a ranking (by Fav Num) higher than "Mike".

NOTE: There are a few things to be aware about this statement.

• This only implements the full CASE statement, it does not implement the "simple" CASE statement.
• At least one when_ and then_ is mandatory otherwise it will emit an error.
• The else_ is also mandatory, unlike the SQL statement in which if the ELSE is omitted it will return a NULL. You can reproduce this via nothing.

Since: 2.1.2

Convert an entity's key into another entity's.

This function is to be used when you change an entity's Id to be that of another entity. For example:

Bar
  barNum Int
Foo
  Id BarId
  fooNum Int

For this example, declare:

instance ToBaseId Foo where
  type BaseEnt Foo = Bar
  toBaseIdWitness = FooKey

Now you're able to write queries such as:

select $
from $ (bar `InnerJoin` foo) -> do
on (toBaseId (foo ^. FooId) ==. bar ^. BarId)
return (bar, foo)

Note: this function may be unsafe to use in conditions not like the one of the example above.

Since: 2.4.3

Class that enables one to use toBaseId to convert an entity's key on a query into another (cf. toBaseId).

Syntax sugar for case_.

Since: 2.1.2

Syntax sugar for case_.

Since: 2.1.2

Syntax sugar for case_.

Since: 2.1.2

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 left fixity.

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
                (InnerJoin (expr (Entity Person))
                           (expr (Entity Follow)))
                (expr (Entity Person))
(p1 `InnerJoin` (f `InnerJoin` p2)) ::
  :: (...) => InnerJoin
                (expr (Entity Person))
                (InnerJoin (expr (Entity Follow))
                           (expr (Entity Person)))

Note that some backends may not support all kinds of JOINs.

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

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).

Phantom type used by orderBy, asc and desc.

Phantom type used by distinctOn and don.

Different kinds of locking clauses supported by locking.

Note that each RDBMS has different locking support. The constructors of this datatype specify only the syntax of the locking mechanism, not its semantics. For example, even though both MySQL and PostgreSQL support ForUpdate, there are no guarantees that they will behave the same.

Since: 2.2.7

Phantom class of data types that are treated as strings by the RDBMS. It has no methods because it's only used to avoid type errors such as trying to concatenate integers.

If you have a custom data type or newtype, feel free to make it an instance of this class.

Since: 2.4.0

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

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

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.

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

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

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

SQL backend for esqueleto using SqlPersistT.

An expression on the SQL backend.

There are many comments describing the constructors of this data type. However, Haddock doesn't like GADTs, so you'll have to read them by hitting "Source".

Constraint synonym for persistent entities whose backend is SqlPersistT.

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).

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

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 ()

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

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)

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

Insert a PersistField for every selected value.

Since: 2.4.2

Insert a PersistField for every selected value, return the count afterward

Apply a PersistField constructor to SqlExpr Value arguments.

Apply extra SqlExpr Value arguments to a PersistField constructor

(Internal) Class that implements the tuple from magic (see fromStart).

valkey i = val . toSqlKey (https://github.com/prowdsponsor/esqueleto/issues/9).

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

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

A more general way to convert instances of ToJSON type class to strict text Text.

Predefined parseJSON. The input JSON looks like {"id": 1, "name": ...}.

The typical usage is:

instance FromJSON (Entity User) where
    parseJSON = entityIdFromJSON

Predefined toJSON. The resulting JSON looks like {"id": 1, "name": ...}.

The typical usage is:

instance ToJSON (Entity User) where
    toJSON = entityIdToJSON

Get list of values corresponding to given entity.

Convenience function for getting a free PersistField instance from a type with JSON instances. The JSON parser used will accept JSON values other that object and arrays. So, if your instance serializes the data to a JSON string, this will still work.

Example usage in combination with toPersistValueJSON:

instance PersistField MyData where
  fromPersistValue = fromPersistValueJSON
  toPersistValue = toPersistValueJSON

Predefined parseJSON. The input JSON looks like {"key": 1, "value": {"name": ...}}.

The typical usage is:

instance FromJSON (Entity User) where
    parseJSON = keyValueEntityFromJSON

Predefined toJSON. The resulting JSON looks like {"key": 1, "value": {"name": ...}}.

The typical usage is:

instance ToJSON (Entity User) where
    toJSON = keyValueEntityToJSON

Convenience function for getting a free PersistField instance from a type with JSON instances.

Example usage in combination with fromPersistValueJSON:

instance PersistField MyData where
  fromPersistValue = fromPersistValueJSON
  toPersistValue = toPersistValueJSON

Get the Keys of all records matching the given criterion.

Curry this to make a convenience function that loads an associated model.

foreign = belongsTo foreignId

Same as belongsTo, but uses getJust and therefore is similarly unsafe.

Like get, but returns the complete Entity.

Example usage

getSpjEntity :: MonadIO m => ReaderT SqlBackend m (Maybe (Entity User))
getSpjEntity = getEntity spjId

mSpjEnt <- getSpjEntity

+----+------+-----+
| id | name | age |
+----+------+-----+
|  1 | SPJ  |  40 |
+----+------+-----+

Same as get, but for a non-null (not Maybe) foreign key. Unsafe unless your database is enforcing that the foreign key is valid.

Example usage

getJustSpj :: MonadIO m => ReaderT SqlBackend m User
getJustSpj = getJust spjId

spj <- getJust spjId

+----+------+-----+
| id | name | age |
+----+------+-----+
|  1 | SPJ  |  40 |
+----+------+-----+

getJustUnknown :: MonadIO m => ReaderT SqlBackend m User
getJustUnknown = getJust unknownId

Same as getJust, but returns an Entity instead of just the record.

Example usage

getJustEntitySpj :: MonadIO m => ReaderT SqlBackend m (Entity User)
getJustEntitySpj = getJustEntity spjId

spjEnt <- getJustEntitySpj

+----+------+-----+
| id | name | age |
+----+------+-----+
|  1 | SPJ  |  40 |
+----+------+-----+

Since: persistent-2.6.1

Like insert, but returns the complete Entity.

Example usage

insertHaskellEntity :: MonadIO m => ReaderT SqlBackend m (Entity User)
insertHaskellEntity = insertEntity $ User "Haskell" 81

haskellEnt <- insertHaskellEntity

+----+---------+-----+
| id |  name   | age |
+----+---------+-----+
|  1 | SPJ     |  40 |
+----+---------+-----+
|  2 | Simon   |  41 |
+----+---------+-----+
|  3 | Haskell |  81 |
+----+---------+-----+

Like insertEntity but just returns the record instead of Entity.

Example usage

insertDaveRecord :: MonadIO m => ReaderT SqlBackend m User
insertDaveRecord = insertRecord $ User "Dave" 50

dave <- insertDaveRecord

+-----+------+-----+
|id   |name  |age  |
+-----+------+-----+
|1    |SPJ   |40   |
+-----+------+-----+
|2    |Simon |41   |
+-----+------+-----+
|3    |Dave  |50   |
+-----+------+-----+

Since: persistent-2.6.1

Check whether there are any conflicts for unique keys with this entity and existing entities in the database.

Returns Nothing if the entity would be unique, and could thus safely be inserted. on a conflict returns the conflicting key

Example usage

mAlanConst <- checkUnique $ User "Alan" 70

mSpjConst <- checkUnique $ User "SPJ" 60

A modification of getBy, which takes the PersistEntity itself instead of a Unique record. Returns a record matching one of the unique keys. This function makes the most sense on entities with a single Unique constructor.

Example usage

mSpjEnt <- getBySpjValue

+----+------+-----+
| id | name | age |
+----+------+-----+
|  1 | SPJ  |  40 |
+----+------+-----+

Insert a value, checking for conflicts with any unique constraints. If a duplicate exists in the database, it is returned as Left. Otherwise, the new 'Key is returned as Right.

Example usage

l1 <- insertBy $ User "SPJ" 20
l2 <- insertBy $ User "XXX" 41
l3 <- insertBy $ User "SPJ" 40
r1 <- insertBy $ User "XXX" 100

Like insertEntity, but returns Nothing when the record couldn't be inserted because of a uniqueness constraint.

Example usage

insertUniqueSpjEntity :: MonadIO m => ReaderT SqlBackend m (Maybe (Entity User))
insertUniqueSpjEntity = insertUniqueEntity $ User "SPJ" 50

mSpjEnt <- insertUniqueSpjEntity

insertUniqueAlexaEntity :: MonadIO m => ReaderT SqlBackend m (Maybe (Entity User))
insertUniqueAlexaEntity = insertUniqueEntity $ User "Alexa" 3

mAlexaEnt <- insertUniqueSpjEntity

+----+-------+-----+
| id | name  | age |
+----+-------+-----+
|  1 | SPJ   |  40 |
+----+-------+-----+
|  2 | Simon |  41 |
+----+-------+-----+
|  3 | Alexa |   3 |
+----+-------+-----+

Since: persistent-2.7.1

Return the single unique key for a record.

Example usage

onlySimonConst :: MonadIO m => ReaderT SqlBackend m (Unique User)
onlySimonConst = onlyUnique $ User "Simon" 999

mSimonConst <- onlySimonConst

Attempt to replace the record of the given key with the given new record. First query the unique fields to make sure the replacement maintains uniqueness constraints.

Return Nothing if the replacement was made. If uniqueness is violated, return a Just with the Unique violation

Since: persistent-1.2.2.0

Commit the current transaction and begin a new one.

Since: persistent-1.2.0

Roll back the current transaction and begin a new one.

Since: persistent-1.2.0

Create the list of columns for the given entity.

Return all of the Sql values associated with the given migration. Calls error if there's a parse error on any migration.

Given a list of old entity definitions and a new EntityDef in val, this creates a Migration to update the old list of definitions with the new one.

Given a Migration, this parses it and returns either a list of errors associated with the migration or a list of migrations to do.

Like parseMigration, but instead of returning the value in an Either value, it calls error on the error values.

Prints a migration.

Runs a migration. If the migration fails to parse or if any of the migrations are unsafe, then this calls error to halt the program.

Same as runMigration, but returns a list of the SQL commands executed instead of printing them to stderr.

Like runMigration, but this will perform the unsafe database migrations instead of erroring out.

Convert a Migration to a list of Text values corresponding to their Sql statements.

Generates sql for limit and offset for postgres, sqlite and mysql.

useful for a backend to implement fieldName by adding escaping

get the SQL string for the field that an EntityField represents Useful for raw SQL queries

Your backend may provide a more convenient fieldName function which does not operate in a Monad

get the SQL string for the table that a PeristEntity represents Useful for raw SQL queries

Your backend may provide a more convenient tableName function which does not operate in a Monad

useful for a backend to implement tableName by adding escaping

Execute a raw SQL statement

Execute a raw SQL statement and return the number of rows it has modified.

Execute a raw SQL statement and return its results as a list. If you do not expect a return value, use of rawExecute is recommended.

If you're using Entitys (which is quite likely), then you must use entity selection placeholders (double question mark, ??). These ?? placeholders are then replaced for the names of the columns that we need for your entities. You'll receive an error if you don't use the placeholders. Please see the Entitys documentation for more details.

You may put value placeholders (question marks, ?) in your SQL query. These placeholders are then replaced by the values you pass on the second parameter, already correctly escaped. You may want to use toPersistValue to help you constructing the placeholder values.

Since you're giving a raw SQL statement, you don't get any guarantees regarding safety. If rawSql is not able to parse the results of your query back, then an exception is raised. However, most common problems are mitigated by using the entity selection placeholder ??, and you shouldn't see any error at all if you're not using Single.

Some example of rawSql based on this schema:

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
Person
    name String
    age Int Maybe
    deriving Show
BlogPost
    title String
    authorId PersonId
    deriving Show
|]

Examples based on the above schema:

getPerson :: MonadIO m => ReaderT SqlBackend m [Entity Person]
getPerson = rawSql "select ?? from person where name=?" [PersistText "john"]

getAge :: MonadIO m => ReaderT SqlBackend m [Single Int]
getAge = rawSql "select person.age from person where name=?" [PersistText "john"]

getAgeName :: MonadIO m => ReaderT SqlBackend m [(Single Int, Single Text)]
getAgeName = rawSql "select person.age, person.name from person where name=?" [PersistText "john"]

getPersonBlog :: MonadIO m => ReaderT SqlBackend m [(Entity Person, Entity BlogPost)]
getPersonBlog = rawSql "select ??,?? from person,blog_post where person.id = blog_post.author_id" []

Minimal working program for PostgreSQL backend based on the above concepts:

{-# LANGUAGE EmptyDataDecls             #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses      #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE QuasiQuotes                #-}
{-# LANGUAGE TemplateHaskell            #-}
{-# LANGUAGE TypeFamilies               #-}

import           Control.Monad.IO.Class  (liftIO)
import           Control.Monad.Logger    (runStderrLoggingT)
import           Database.Persist
import           Control.Monad.Reader
import           Data.Text
import           Database.Persist.Sql
import           Database.Persist.Postgresql
import           Database.Persist.TH

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
Person
    name String
    age Int Maybe
    deriving Show
|]

conn = "host=localhost dbname=new_db user=postgres password=postgres port=5432"

getPerson :: MonadIO m => ReaderT SqlBackend m [Entity Person]
getPerson = rawSql "select ?? from person where name=?" [PersistText "sibi"]

liftSqlPersistMPool y x = liftIO (runSqlPersistMPool y x)

main :: IO ()
main = runStderrLoggingT $ withPostgresqlPool conn 10 $ liftSqlPersistMPool $ do
         runMigration migrateAll
         xs <- getPerson
         liftIO (print xs)

Get a connection from the pool, run the given action, and then return the connection to the pool.

Note: This function previously timed out after 2 seconds, but this behavior was buggy and caused more problems than it solved. Since version 2.1.2, it performs no timeout checks.

Create a connection and run sql queries within it. This function automatically closes the connection on it's completion.

Example usage

{-# LANGUAGE GADTs #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies#-}
{-# LANGUAGE TemplateHaskell#-}
{-# LANGUAGE QuasiQuotes#-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import Control.Monad.IO.Class  (liftIO)
import Control.Monad.Logger
import Conduit
import Database.Persist
import Database.Sqlite
import Database.Persist.Sqlite
import Database.Persist.TH

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
Person
  name String
  age Int Maybe
  deriving Show
|]

openConnection :: LogFunc -> IO SqlBackend
openConnection logfn = do
 conn <- open "/home/sibi/test.db"
 wrapConnection conn logfn

main :: IO ()
main = do
  runNoLoggingT $ runResourceT $ withSqlConn openConnection (\backend ->
                                      flip runSqlConn backend $ do
                                        runMigration migrateAll
                                        insert_ $ Person "John doe" $ Just 35
                                        insert_ $ Person "Divya" $ Just 36
                                        (pers :: [Entity Person]) <- selectList [] []
                                        liftIO $ print pers
                                        return ()
                                     )

Migrating: CREATE TABLE "person"("id" INTEGER PRIMARY KEY,"name" VARCHAR NOT NULL,"age" INTEGER NULL)
[Entity {entityKey = PersonKey {unPersonKey = SqlBackendKey {unSqlBackendKey = 1}}, entityVal = Person {personName = "John doe", personAge = Just 35}},Entity {entityKey = PersonKey {unPersonKey = SqlBackendKey {unSqlBackendKey = 2}}, entityVal = Person {personName = "Hema", personAge = Just 36}}]

Useful for running a read query against a backend with unknown capabilities.

Useful for running a read query against a backend with read and write capabilities.

Useful for running a write query against an untagged backend with unknown capabilities.