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

(Internal) Create a case statement.

Since: 2.1.1

(Internal) Create a custom binary operator. You should not use this function directly since its type is very general, you should always use it with an explicit type signature. For example:

(==.) :: SqlExpr (Value a) -> SqlExpr (Value a) -> SqlExpr (Value Bool)
(==.) = unsafeSqlBinOp " = "

In the example above, we constraint the arguments to be of the same type and constraint the result to be a boolean value.

Similar to unsafeSqlBinOp, but may also be applied to composite keys. Uses the operator given as the second argument whenever applied to composite keys.

Usage example:

(==.) :: SqlExpr (Value a) -> SqlExpr (Value a) -> SqlExpr (Value Bool)
(==.) = unsafeSqlBinOpComposite " = " " AND "

Persistent has a hack for implementing composite keys (see ECompositeKey doc for more details), so we're forced to use a hack here as well. We deconstruct ERaw values based on two rules:

• If it is a single placeholder, then it's assumed to be coming from a PersistList and thus its components are separated so that they may be applied to a composite key.
• If it is not a single placeholder, then it's assumed to be a foreign (composite or not) key, so we enforce that it has no placeholders and split it on the commas.

(Internal) A raw SQL value. The same warning from unsafeSqlBinOp applies to this function as well.

(Internal) An explicit SQL type cast using CAST(value as type). See unsafeSqlBinOp for warnings.

(Internal) A raw SQL function. Once again, the same warning from unsafeSqlBinOp applies to this function as well.

(Internal) An unsafe SQL function to extract a subfield from a compound field, e.g. datetime. See unsafeSqlBinOp for warnings.

Since: 1.3.6.

A ORDER BY clause.

(Internal) Execute an esqueleto SELECT SqlQuery inside persistent's SqlPersistT monad.

(Internal) Run a Source of rows.

(Internal) Execute an esqueleto statement inside persistent's SqlPersistT monad.

(Internal) Pretty prints a SqlQuery into a SQL query.

Note: if you're curious about the SQL query being generated by esqueleto, instead of manually using this function (which is possible but tedious), you may just turn on query logging of persistent.

(Internal) Mode of query being converted by toRawSql.

List of identifiers already in use and supply of temporary identifiers.

Information needed to escape and use identifiers.

(Internal) Class for mapping results coming from SqlQuery into actual results.

This looks very similar to RawSql, and it is! However, there are some crucial differences and ultimately they're different classes.

(Internal) Coerce a value's type from 'SqlExpr (Value a)' to 'SqlExpr (Value b)'. You should not use this function unless you know what you're doing!

(Internal) Coerce a value's type from 'SqlExpr (ValueList a)' to 'SqlExpr (Value a)'. Does not work with empty lists.