persistent- Type-safe, multi-backend data serialization.
Safe HaskellSafe-Inferred



This module defines the Persistent entity syntax used in the quasiquoter to generate persistent entities.

The basic structure of the syntax looks like this:

    fieldName      FieldType
    otherField     String
    nullableField  Int       Maybe

You start an entity definition with the table name, in this case, TableName. It's followed by a list of fields on the entity, which have the basic form fieldName FieldType. You can indicate that a field is nullable with Maybe at the end of the type.

persistent automatically generates an ID column for you, if you don't specify one, so the above table definition corresponds to the following SQL:

CREATE TABLE table_name (
    id                SERIAL PRIMARY KEY,
    field_name        field_type NOT NULL,
    other_field       varchar    NOT NULL,
    nullable_field    int NULL

Note that the exact SQL that is generated can be customized using the PersistSettings that are passed to the parse function.

It generates a Haskell datatype with the following form:

data TableName = TableName
    { tableNameFieldName :: FieldType
    , tableNameOtherField :: String
    , tableNameNullableField :: Maybe Int

As with the SQL generated, the specifics of this are customizable. See the Database.Persist.TH module for details.


You can add a deriving clause to a table, and the generated Haskell type will have a deriving clause with that. Unlike normal Haskell syntax, you don't need parentheses or commas to separate the classes, and you can even have multiple deriving clauses.

    name String
    age  Int
    deriving Eq Show
    deriving Ord

Unique Keys

You can define a uniqueness key on a table with the following format:

   name String
   age  Int

   UniqueUserName name

This will put a unique index on the user table and the name field.

Setting defaults

You can use a default=${sql expression} clause to set a default for a field. The thing following the = is interpreted as SQL that will be put directly into the table definition.

    name    Text
    admin   Bool default=false

This creates a SQL definition like this:

  admin   BOOL DEFAULT=false

A restriction here is that you still need to provide a value when performing an insert, because the generated Haskell type has the form:

data User = User
    { userName :: Text
    , userAdmin :: Bool

You can work around this by using a 'Maybe Bool' and supplying Nothing by default.

Note: Persistent determines whether or not to migrate a column's default value by comparing the exact string found in your models file with the one returned by the database. If a database canonicalizes the SQL FALSE from your models file to false in the database, Persistent will think the default value needs to be migrated and attempt a migration each time you start your app.

To workaround this, find the exact SQL your DBMS uses for the default value. For example, using postgres:

psql database_name # Open postgres

\d+ table_name -- describe the table schema
created       | timestamp without time zone | not null default now()

Then use the listed default value SQL inside your models file.

Custom ID column

If you don't want to use the default ID column type of Int64, you can set a custom type with an Id field. This User has a Text ID.

    Id   Text
    name Text
    age  Int

If you do this, it's a good idea to set a default for the ID. Otherwise, you will need to use insertKey instead of insert when performing inserts.

insertKey (UserKey "Hello world!") (User Bob 32)

If you attempt to do insert (User Bob 32), then you will receive a runtime error because the SQL database doesn't know how to make an ID for you anymore. So instead just use a default expression, like this:

    Id      Text default=generate_user_id()
    name    Text
    age     Int

Custom Primary Keys

Sometimes you don't want to have an ID column, and you want a different sort of primary key. This is a table that stores unique email addresses, and the email is the primary key. We store the first and second part (eg first@second) separately.

    firstPart   Text
    secondPart  Text

    Primary firstPart secondPart

This creates a table with the following form:

    first_part  varchar,
    second_part varchar,

    PRIMARY KEY (first_part, second_part)

Since the primary key for this table is part of the record, it's called a "natural key" in the SQL lingo. As a key with multiple fields, it is also a "composite key."

You can specify a Primary key with a single field, too.

Overriding SQL

You can use a sql=custom annotation to provide some customization on the entity and field. For example, you might prefer to name a table differently than what persistent will do by default. You may also prefer to name a field differently.

User sql=big_user_table
    fullName    String sql=name
    age         Int

This will alter the generated SQL to be:

CREATE TABEL big_user_table (
    name    VARCHAR,
    age     INT

Customizing Types/Tables

JSON instances

You can automatically get ToJSON and FromJSON instances for any entity by adding json to the entity line:

Person json
    name Text

Requires {-# LANGUAGE FlexibleInstances #-}

Customizable by using mpsEntityJSON * *

Changing table/collection name

Person sql=peoples
    name Text

Change table/collection key definition (field name and/or type, persistent >= 2.1)

Id defines the column to use to define the key of the entity. Without type, the default backend key type will be used. You can change its database name using the sql attributes :

   Id         sql=my_id_name
   phone Text

With a Haskell type, the corresponding type is used. Note that you'll need to use default= to tell it what to do on insertion.

   Id    Day default=CURRENT_DATE
   phone Text

default= works for SQL databases, and is backend specific. For MongoDB currently one always needs to create the key on the application side and use insertKey. insert will not work correctly. Sql backends can also do this if default does not work.

sqltype can also be used to specify a different database type

    Id String sqltype=varchar(3) sql=code

Composite key (using multiple columns) can also be defined using Primary.

sql= also works for setting the names of unique indexes.

  name Text
  phone Text
  UniquePersonPhone phone sql=UniqPerPhone

This makes a unique index requiring phone to be unique across Person rows. Ordinarily Persistent will generate a snake-case index name from the capitalized name provided such that UniquePersonPhone becomes unique_person_phone. However, we provided a sql= so the index name in the database will instead be UniqPerPhone. Keep in mind sql= and ! attrs must come after the list of fields in front of the index name in the quasi-quoter.

Customizing Fields

Nullable Fields

As illustrated in the example at the beginning of this page, we are able to represent nullable fields by including Maybe at the end of the type declaration:

    fieldName      FieldType
    otherField     String
    nullableField  Int       Maybe

Alternatively we can specify the keyword nullable:

    fieldName      FieldType
    otherField     String
    nullableField  Int       nullable

However the difference here is in the first instance the Haskell type will be 'Maybe Int', but in the second it will be Int. Be aware that this will cause runtime errors if the database returns NULL and the PersistField instance does not handle PersistNull.

If you wish to define your Maybe types in a way that is similar to the actual Haskell definition, you can define 'Maybe Int' like so:

    fieldName      FieldType
    otherField     String
    nullableField  (Maybe Int)

However, note, the field _must_ be enclosed in parenthesis.


By default, Persistent maps the Haskell types you specify in the Models DSL to an appropriate SQL type in the database (refer to the section "Conversion table (migrations)" for the default mappings). Using the sqltype= option, you can customize the SQL type Persistent uses for your column. Use cases include:

  • Interacting with an existing database whose column types don't match Persistent's defaults.
  • Taking advantage of a specific SQL type's features
  • e.g. Using an equivalent type that has better space or performance characteristics

To use this setting, add the sqltype= option after declaring your field name and type:

    username Text sqltype=varchar(255)


By default the records created by persistent have strict fields. You can prefix a field name with ~ to make it lazy (or ! to make it strict).


The QuasiQuoter allows you to provide arbitrary attributes to an entity or field. This can be used to extend the code in ways that the library hasn't anticipated. If you use this feature, we'd definitely appreciate hearing about it and potentially supporting your use case directly!

User !funny
    field   String  !sad
    good    Dog     !sogood

We can see the attributes using the entityAttrs field and the fieldAttrs field.

userAttrs = do
    let userDefinition = entityDef (Proxy :: Proxy User)
    let userAttributes = entityAttrs userDefinition
    let fieldAttributes = map fieldAttrs (entityFields userDefinition)
    print userAttributes
-- ["funny"]
    print fieldAttributes
-- [["sad"],["sogood"]]


Introduced with persistent-template 1.2.0. The purpose of this attribute is to mark a field which will be entirely ignored by the normal processing, but retained in the database definition for purposes of migration. This means, in SQL, a column will not be flagged for removal by the migration scripts, even though it is not used in your code. This is useful for phasing out usage of a column before entirely removing it, or having columns which are needed by other tools but not by Persistent.

    name Text
    age Int
    unusedField ByteString Maybe MigrationOnly

Note that you almost certainly want to either mark the field as Maybe or provide a default value, otherwise insertions will fail.


This is intended to be used as part of a deprecation of a field, after MigrationOnly has been used usually. This works somewhat as a superset of the functionality of MigrationOnly. In addition, the field will be removed from the database if it is present. Note that this is a destructive change which you are marking as safe.


Migration will remove any manual constraints from your tables. Exception: constraints whose names begin with the string __manual_ (which starts with two underscores) will be preserved.

Foreign Keys

If you define an entity and want to refer to it in another table, you can use the entity's Id type in a column directly.

    name    Text

    name    Text
    owner   PersonId

This automatically creates a foreign key reference from Dog to Person. The foreign key constraint means that, if you have a PersonId on the Dog, the database guarantees that the corresponding Person exists in the database. If you try to delete a Person out of the database that has a Dog, you'll receive an exception that a foreign key violation has occurred.


You can use the constraint= attribute to override the constraint name used in migrations. This is useful particularly when the automatically generated constraint names exceed database limits (e.g. MySQL does not allow constraint names longer than 64 characters).

  name Text

  veryLongTableNameId VeryLongTableNameId constraint=short_foreign_key

OnUpdate and OnDelete

These options affects how a referring record behaves when the target record is changed. There are several options:

  • Restrict - This is the default. It prevents the action from occurring.
  • Cascade - this copies the change to the child record. If a parent record is deleted, then the child record will be deleted too.
  • SetNull - If the parent record is modified, then this sets the reference to NULL. This only works on Maybe foreign keys.
  • SetDefault - This will set the column's value to the default for the column, if specified.

To specify the behavior for a reference, write OnUpdate or OnDelete followed by the action.

    -- If the referred Foo is deleted or updated, then this record will
    -- also be deleted or updated.
    fooId   FooId   OnDeleteCascade OnUpdateCascade

    -- If the referred Bar is deleted, then we'll set the reference to
    -- Nothing. If the referred Bar is updated, then we'll cascade the
    -- update.
    barId   BarId Maybe     OnDeleteSetNull OnUpdateCascade

    -- If the referred Baz is deleted, then we set to the default ID.
    bazId   BazId   OnDeleteSetDefault  default=1

Let's demonstrate this with a shopping cart example.

    name    Text

    user    UserId Maybe

    cartId  CartId
    itemId  ItemId

    name    Text
    price   Int

Let's consider how we want to handle deletions and updates. If a User is deleted or update, then we want to cascade the action to the associated Cart.

    user    UserId Maybe OnDeleteCascade OnUpdateCascade

If an Item is deleted, then we want to set the CartItem to refer to a special "deleted item" in the database. If a Cart is deleted, though, then we just want to delete the CartItem.

    cartId CartId   OnDeleteCascade
    itemId ItemId   OnDeleteSetDefault default=1

Foreign keyword

The above example is a "simple" foreign key. It refers directly to the Id column, and it only works with a non-composite primary key. We can define more complicated foreign keys using the Foreign keyword.

A pseudo formal syntax for Foreign is:

Foreign $(TargetEntity) [$(cascade-actions)] $(constraint-name) $(columns) [ $(references) ]

columns := column0 [column1 column2 .. columnX]
references := References $(target-columns)
target-columns := target-column0 [target-column1 target-columns2 .. target-columnX]

Columns are the columns as defined on this entity. target-columns are the columns as defined on the target entity.

Let's look at some examples.

Composite Primary Key References

The most common use for this is to refer to a composite primary key. Since composite primary keys take up more than one column, we can't refer to them with a single persistent column.

    firstPart   Text
    secondPart  Text
    Primary firstPart secondPart

    name            Text
    emailFirstPart  Text
    emailSecondPart Text

    Foreign Email fk_user_email emailFirstPart emailSecondPart

If you omit the References keyword, then it assumes that the foreign key reference is for the target table's primary key. If we wanted to be fully redundant, we could specify the References keyword.

    Foreign Email fk_user_email emailFirstPart emailSecondPart References firstPart secondPart

We can specify delete/cascade behavior directly after the target table.

    Foreign Email OnDeleteCascade OnUpdateCascade fk_user_email emailFirstPart emailSecondPart

Now, if the email is deleted or updated, the user will be deleted or updated to match.

Non-Primary Key References

SQL database backends allow you to create a foreign key to any column(s) with a Unique constraint. Persistent does not check this, because you might be defining your uniqueness constraints outside of Persistent. To do this, we must use the References keyword.

    name    Text
    email   Text

    UniqueEmail email

    content Text
    sentTo  Text

    Foreign User fk_noti_user sentTo References email

If the target uniqueness constraint has multiple columns, then you must specify them independently.

    name            Text
    emailFirst      Text
    emailSecond     Text

    UniqueEmail emailFirst emailSecond

    content         Text
    sentToFirst     Text
    sentToSecond    Text

    Foreign User fk_noti_user sentToFirst sentToSecond References emailFirst emailSecond

Documentation Comments

The quasiquoter supports ordinary comments with -- and #. Since persistent-, it also supports documentation comments. The grammar for documentation comments is similar to Haskell's Haddock syntax, with a few restrictions:

  1. Only the -- | form is allowed.
  2. You must put a space before and after the | pipe character.
  3. The comment must be indented at the same level as the entity or field it documents.

An example of the field documentation is:

-- | I am a doc comment for a User. Users are important
-- | to the application, and should be treasured.
    -- | Users have names. Call them by names.
    name String
    -- | A user can be old, or young, and we care about
    -- | this for some reason.
    age Int

The documentation is present on the entityComments field on the EntityDef for the entity:

>>> let userDefinition = entityDef (Proxy :: Proxy User)
>>> entityComments userDefinition
"I am a doc comment for a User. Users are importantnto the application, and should be treasured.n"

Likewise, the field documentation is present in the fieldComments field on the FieldDef present in the EntityDef:

>>> let userFields = entityFields userDefinition
>>> let comments = map fieldComments userFields
>>> mapM_ putStrLn comments
"Users have names. Call them by names."
"A user can be old, or young, and we care aboutnthis for some reason."

Since persistent-, documentation comments are included in documentation generated using Haddock if mpsEntityHaddocks is enabled (defaults to False). persistent backends can also use this to generate SQL COMMENTs, which are useful for a database perspective, and you can use the @persistent-documentation@ library to render a Markdown document of the entity definitions.

Sum types

Field level

You'll frequently want to store an enum of values in your database. For example, you might describe a Person's employment status as being Employed, Unemployed, or Retired. In Haskell this is represented with a sum type, and Persistent provides a Template Haskell function to marshall these values to and from the database:

-- @Employment.hs

module Employment where

import Database.Persist.TH
import Prelude

data Employment = Employed | Unemployed | Retired
    deriving (Show, Read, Eq)
derivePersistField Employment

derivePersistField stores sum type values as strins in the database. While not as efficient as using integers, this approach simplifies adding and removing values from your enumeration.

Due to the GHC Stage Restriction, the call to the Template Haskell function derivePersistField must be in a separate module than where the generated code is used.

Note: If you created a new module, make sure add it to the exposed-modules section of your Cabal file.

Use the module by importing it into your Model.hs file:

-- @Model.hs
import Employment

and use it in the models DSL:

    employment Employment

You can export the Employment module from Import to use it across your app:

-- @Import.hs
import Employment as Import


NOTE: This feature is deprecated as of version 2.14 and will be removed in 2.15 (unless there are many complaints).

The tests for this feature demonstrate their usage. Note the use of the sign + in front of the entity name.

The schema in the test is reproduced here:

share [mkPersist persistSettings, mkMigrate "sumTypeMigrate"] [persistLowerCase|
    brand T.Text
    make T.Text
    model T.Text
    bicycle BicycleId
    car CarId

Let's check out the definition of the Haskell type Vehicle. Using ghci, we can query for :info Vehicle:

>>> :i Vehicle
type Vehicle = VehicleGeneric SqlBackend
        -- Defined at .../Projects/persistent/persistent-test/src/SumTypeTest.hs:26:1
>>> :i VehicleGeneric
type role VehicleGeneric nominal
data VehicleGeneric backend
  = VehicleBicycleSum (Key (BicycleGeneric backend))
  | VehicleCarSum (Key (CarGeneric backend))
        -- Defined at .../persistent/persistent-test/src/SumTypeTest.hs:26:1
-- lots of instances follow...

A VehicleGeneric has two constructors:

  • VehicleBicycleSum with a Key (BicycleGeneric backend) field
  • VehicleCarSum with a Key (CarGeneric backend) field

The Bicycle and Car are typical persistent entities.

This generates the following SQL migrations (formatted for readability):

CREATE TABLE "bicycle" (
    "id"        INTEGER PRIMARY KEY,
    "brand"     VARCHAR NOT NULL

    "id"        INTEGER PRIMARY KEY,
    "make"      VARCHAR NOT NULL,
    "model"     VARCHAR NOT NULL

CREATE TABLE "vehicle"(
    "id"        INTEGER PRIMARY KEY,
    "bicycle"   INTEGER NULL REFERENCES "bicycle",
    "car"       INTEGER NULL REFERENCES "car"

The vehicle table contains a nullable foreign key reference to both the bicycle and the car tables.

A SQL query that grabs all the vehicles from the database looks like this (note the ?? is for the persistent raw SQL query functions):

SELECT ??, ??, ??
FROM vehicle
    ON =
LEFT JOIN bicycle
    ON vehicle.bicycle =

If we use the above query with rawSql, we'd get the following result:

    :: SqlPersistM
        [ ( Entity Vehicle
          , Maybe (Entity Bicycle)
          , Maybe (Entity Car)

This result has some post-conditions that are not guaranteed by the types *or* the schema. The constructor for Entity Vehicle is going to determine which of the other members of the tuple is Nothing. We can convert this to a friendlier domain model like this:

data Vehicle'
    = Car' Text Text
    | Bike Text

check = do
    result <- getVehicles
    pure (map convert result)

    :: (Entity Vehicle, Maybe (Entity Bicycle), Maybe (Entity Car))
    -> Vehicle'
convert (Entity _ (VehicycleBicycleSum _), Just (Entity _ (Bicycle brand)), _) =
    Bike brand
convert (Entity _ (VehicycleCarSum _), _, Just (Entity _ (Car make model))) =
    Car make model
convert _ =
    error "The database preconditions have been violated!"

Times with timezones

Storing times with timezones in one type in databases is not possible, although it seems that it should be possible (timezone and timezonetz in PostgreSQL). That's why starting with persistent 2.0, all times will be mapped to UTCTime. If you need to store timezone information along with times in a database, store the timezone in a second field. Here are some links about the topic with further information:

Conversion table (migrations)

Here are the conversions between Haskell types and database types:

RationalNUMERIC(22, 12)DECIMAL(32,20)*Unsupported*NUMERIC(32,20)


* Support for ZonedTime was dropped in persistent 2.0. UTCTime can be used with timestamp without timezone and timestamp with timezone in PostgreSQL. See also the section "Times with timezones".

** The default resolution for TIME and DATETIME in MySQL is one second. As of MySQL version 5.6.4, and persistent-mysql-2.6.2, fractional seconds are handled correctly if you declare an explicit precision by using sqltype. For example, appending sqltype=TIME(6) to a TimeOfDay field definition will give microsecond resolution.

Compatibility tables


Haskell typeCompatible MySQL types
Word64 DoubleTiny,Short,Int24,Long,LongLong Float,Double,Decimal,NewDecimal,Tiny,Short,Int24,Long
Ratio IntegerFloat,Double,Decimal,NewDecimal,Tiny,Short,Int24,Long,LongLong

* When Word size is 64bit

** Utf8 only

Unsupported types:

Not currently supported
Scientific #225

See MySQL.Simple.Result.



parse :: PersistSettings -> Text -> [UnboundEntityDef] Source #

Parses a quasi-quoted syntax into a list of entity definitions.


Getters and Setters

getPsToDBName :: PersistSettings -> Text -> Text Source #

Retrieve the function in the PersistSettings that modifies the names into database names.


setPsToDBName :: (Text -> Text) -> PersistSettings -> PersistSettings Source #

Set the name modification function that translates the QuasiQuoted names for use in the database.


setPsToFKName :: (EntityNameHS -> ConstraintNameHS -> Text) -> PersistSettings -> PersistSettings Source #

Set a custom function used to create the constraint name for a foreign key.


setPsUseSnakeCaseForeignKeys :: PersistSettings -> PersistSettings Source #

A preset configuration function that puts an underscore between the entity name and the constraint name when creating a foreign key constraint name


setPsUseSnakeCaseForiegnKeys :: PersistSettings -> PersistSettings Source #

Deprecated: use the correctly spelled, equivalent, setPsUseSnakeCaseForeignKeys instead

getPsStrictFields :: PersistSettings -> Bool Source #

Retrieve whether or not the PersistSettings will generate code with strict fields.


setPsStrictFields :: Bool -> PersistSettings -> PersistSettings Source #

Set whether or not the PersistSettings will make fields strict.


getPsIdName :: PersistSettings -> Text Source #

Retrieve the default name of the id column.


setPsIdName :: Text -> PersistSettings -> PersistSettings Source #

Set the default name of the id column.