persistent- Type-safe, multi-backend data serialization.

Safe HaskellNone




API for database actions. The API deals with fields and entities. In SQL, a field corresponds to a column, and should be a single non-composite value. An entity corresponds to a SQL table, so an entity is a collection of fields.



data SqlType Source

A SQL data type. Naming attempts to reflect the underlying Haskell datatypes, eg SqlString instead of SqlVarchar. Different SQL databases may have different translations for these types.

class PersistEntity val whereSource

A single database entity. For example, if writing a blog application, a blog entry would be an entry, containing fields such as title and content.

Associated Types

data EntityField val :: * -> *Source

Parameters: val and datatype of the field

type PersistEntityBackend val Source

data Unique val Source

Unique keys in existence on this entity.

class MonadIO m => PersistStore m whereSource

Associated Types

type PersistMonadBackend m Source


insert :: (PersistMonadBackend m ~ PersistEntityBackend val, PersistEntity val) => val -> m (Key val)Source

Create a new record in the database, returning an automatically created key (in SQL an auto-increment id).

insert_ :: (PersistMonadBackend m ~ PersistEntityBackend val, PersistEntity val) => val -> m ()Source

Same as insert, but doesn't return a Key.

insertKey :: (PersistMonadBackend m ~ PersistEntityBackend val, PersistEntity val) => Key val -> val -> m ()Source

Create a new record in the database using the given key.

repsert :: (PersistMonadBackend m ~ PersistEntityBackend val, PersistEntity val) => Key val -> val -> m ()Source

Put the record in the database with the given key. Unlike replace, if a record with the given key does not exist then a new record will be inserted.

replace :: (PersistMonadBackend m ~ PersistEntityBackend val, PersistEntity val) => Key val -> val -> m ()Source

Replace the record in the database with the given key. Note that the result is undefined if such record does not exist, so you must use 'insertKey or repsert in these cases.

delete :: (PersistMonadBackend m ~ PersistEntityBackend val, PersistEntity val) => Key val -> m ()Source

Delete a specific record by identifier. Does nothing if record does not exist.

get :: (PersistMonadBackend m ~ PersistEntityBackend val, PersistEntity val) => Key val -> m (Maybe val)Source

Get a record by identifier, if available.

class PersistStore m => PersistUnique m whereSource

Queries against unique keys (other than the id).

Please read the general Persistent documentation to learn how to create Unique keys. SQL backends automatically create uniqueness constraints, but for MongoDB you must place a unique index on the field.


getBy :: (PersistEntityBackend val ~ PersistMonadBackend m, PersistEntity val) => Unique val -> m (Maybe (Entity val))Source

Get a record by unique key, if available. Returns also the identifier.

deleteBy :: (PersistEntityBackend val ~ PersistMonadBackend m, PersistEntity val) => Unique val -> m ()Source

Delete a specific record by unique key. Does nothing if no record matches.

insertUnique :: (PersistEntityBackend val ~ PersistMonadBackend m, PersistEntity val) => val -> m (Maybe (Key val))Source

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

newtype ZT Source

Avoid orphan instances.


ZT ZonedTime 


insertBy :: (PersistEntity v, PersistStore m, PersistUnique m, PersistMonadBackend m ~ PersistEntityBackend v) => v -> m (Either (Entity v) (Key v))Source

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.

getByValue :: (PersistEntity v, PersistUnique m, PersistEntityBackend v ~ PersistMonadBackend m) => v -> m (Maybe (Entity v))Source

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

getJust :: (PersistStore m, PersistEntity val, Show (Key val), PersistMonadBackend m ~ PersistEntityBackend val) => Key val -> m valSource

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

belongsTo :: (PersistStore m, PersistEntity ent1, PersistEntity ent2, PersistMonadBackend m ~ PersistEntityBackend ent2) => (ent1 -> Maybe (Key ent2)) -> ent1 -> m (Maybe ent2)Source

belongsToJust :: (PersistStore m, PersistEntity ent1, PersistEntity ent2, PersistMonadBackend m ~ PersistEntityBackend ent2) => (ent1 -> Key ent2) -> ent1 -> m ent2Source

same as belongsTo, but uses getJust and therefore is similarly unsafe

checkUnique :: (PersistEntityBackend val ~ PersistMonadBackend m, PersistEntity val, PersistUnique m) => val -> m BoolSource

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

Returns True if the entity would be unique, and could thus safely be inserted; returns False on a conflict.

newtype KeyBackend backend entity Source




unKey :: PersistValue


Eq (KeyBackend backend entity) 
Ord (KeyBackend backend entity) 
Read (KeyBackend backend entity) 
Show (KeyBackend backend entity) 
ToJSON (KeyBackend backend entity) 
FromJSON (KeyBackend backend entity) 
PathPiece (KeyBackend SqlBackend entity) 
PersistField (KeyBackend backend entity) 

type Key val = KeyBackend (PersistEntityBackend val) valSource

Helper wrapper, equivalent to Key (PersistEntityBackend val) val.

Since 1.1.0

data Entity entity Source

Datatype that represents an entity, with both its key and its Haskell representation.

When using the an SQL-based backend (such as SQLite or PostgreSQL), an Entity may take any number of columns depending on how many fields it has. In order to reconstruct your entity on the Haskell side, persistent needs all of your entity columns and in the right order. Note that you don't need to worry about this when using persistent's API since everything is handled correctly behind the scenes.

However, if you want to issue a raw SQL command that returns an Entity, then you have to be careful with the column order. While you could use SELECT Entity.* WHERE ... and that would work most of the time, there are times when the order of the columns on your database is different from the order that persistent expects (for example, if you add a new field in the middle of you entity definition and then use the migration code -- persistent will expect the column to be in the middle, but your DBMS will put it as the last column). So, instead of using a query like the one above, you may use rawSql (from the Database.Persist.GenericSql module) with its /entity selection placeholder/ (a double question mark ??). Using rawSql the query above must be written as SELECT ?? WHERE ... Then rawSql will replace ?? with the list of all columns that we need from your entity in the right order. If your query returns two entities (i.e. (Entity backend a, Entity backend b)), then you must you use SELECT ??, ?? WHERE ..., and so on.




entityKey :: Key entity
entityVal :: entity


Eq entity => Eq (Entity entity) 
Ord entity => Ord (Entity entity) 
Read entity => Read (Entity entity) 
Show entity => Show (Entity entity) 
PersistEntity a => RawSql (Entity a) 



class PersistConfig c whereSource

Represents a value containing all the configuration options for a specific backend. This abstraction makes it easier to write code that can easily swap backends.

Associated Types

type PersistConfigBackend c :: (* -> *) -> * -> *Source

type PersistConfigPool c Source


loadConfig :: Value -> Parser cSource

Load the config settings from a Value, most likely taken from a YAML config file.

applyEnv :: c -> IO cSource

Modify the config settings based on environment variables.

createPoolConfig :: c -> IO (PersistConfigPool c)Source

Create a new connection pool based on the given config settings.

runPool :: (MonadBaseControl IO m, MonadIO m) => c -> PersistConfigBackend c m a -> PersistConfigPool c -> m aSource

Run a database action by taking a connection from the pool.