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esqueleto Database.Esqueleto 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 subSelect).
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). -
esqueleto Database.Esqueleto.Experimental 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 subSelect).
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). -
esqueleto Database.Esqueleto.Internal.Internal 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 subSelect).
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). -
esqueleto Database.Esqueleto.Legacy 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 subSelect).
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). -
esqueleto Database.Esqueleto.Internal.Internal No documentation available.
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esqueleto Database.Esqueleto Get just the first record for the criterion.
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esqueleto Database.Esqueleto Get the Keys of all records matching the given criterion. For an example, see selectList.
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esqueleto Database.Esqueleto Get the Keys of all records matching the given criterion.
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esqueleto Database.Esqueleto Execute an esqueleto SELECT query inside persistent's SqlPersistT monad and return the first entry wrapped in a Maybe. @since 3.5.1.0
Example usage
firstPerson :: MonadIO m => SqlPersistT m (Maybe (Entity Person)) firstPerson = selectOne $ do person <- from $ table @Person return person
The above query is equivalent to a select combined with limit but you would still have to transform the results from a list:firstPerson :: MonadIO m => SqlPersistT m [Entity Person] firstPerson = select $ do person <- from $ table @Person limit 1 return person
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esqueleto Database.Esqueleto Execute an esqueleto SELECT query inside persistent's SqlPersistT monad and return a Source of rows.
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