esqueleto/README.md

# Esqueleto

Esqueleto is a bare bones, type-safe EDSL for SQL queries that works with
unmodified persistent SQL backends. The name of this library means "skeleton"
in Portuguese and contains all three SQL letters in the correct order =). It
was inspired by Scala's Squeryl but created from scratch. Its language closely
resembles SQL. Currently, SELECTs, UPDATEs, INSERTs and DELETEs are supported.

In particular, esqueleto is the recommended library for type-safe JOINs on
persistent SQL backends. (The alternative is using raw SQL, but that's error
prone and does not offer any composability.). For more information read
[esqueleto](http://hackage.haskell.org/package/esqueleto).

## Setup

If you're already using `persistent`, then you're ready to use
`esqueleto`, no further setup is needed.  If you're just
starting a new project and would like to use `esqueleto`, take
a look at `persistent`'s [book](http://www.yesodweb.com/book/persistent) first
 to learn how to define your schema.

If you need to use `persistent`'s default support for queries
as well, either import it qualified:

```haskell
-- For a module that mostly uses esqueleto.
import Database.Esqueleto
import qualified Database.Persistent as P
```

or import `esqueleto` itself qualified:

```haskell
-- For a module that uses esqueleto just on some queries.
import Database.Persistent
import qualified Database.Esqueleto as E
```

Other than identifier name clashes, `esqueleto` does not
conflict with `persistent` in any way.


## Goals

The main goals of `esqueleto` are:

- Be easily translatable to SQL. (You should be able to know exactly how the SQL query will end up.)
- Support the most widely used SQL features.
- Be as type-safe as possible.

It is _not_ a goal to be able to write portable SQL.
We do not try to hide the differences between DBMSs from you


## Introduction

For the following examples, we'll use this example schema:

```haskell
share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persist|
  Person
    name String
    age Int Maybe
    deriving Eq Show
  BlogPost
    title String
    authorId PersonId
    deriving Eq Show
  Follow
    follower PersonId
    followed PersonId
    deriving Eq Show
|]
```

## Select

Most of `esqueleto` was created with `SELECT` statements in
mind, not only because they're the most common but also
because they're the most complex kind of statement.  The most
simple kind of `SELECT` would be:

```haskell
putPersons :: SqlPersist m ()
putPersons = do
  people <- select $
              from $ \person -> do
              return person
  liftIO $ mapM_ (putStrLn . personName . entityVal) people
```

which generates this SQL:

```sql
SELECT *
FROM Person
```

`esqueleto` knows that we want an `Entity Person` just because of the `personName` that is
printed.

## Where

Filtering by `PersonName``

```haskell
select $
from $ \p -> do
where_ (p ^. PersonName ==. val "John")
return p
```

which generates this SQL:

```sql
SELECT *
FROM Person
WHERE Person.name = "John"
```

The `(^.)` operator is used to project a field from an entity.
The field name is the same one generated by `persistent`s
Template Haskell functions.  We use `val` to lift a constant
Haskell value into the SQL query.

Another example:

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

```haskell
select $
from $ \p -> do
where_ (p ^. PersonAge >=. just (val 18))
return p
```

which generates this SQL:

```sql
SELECT *
FROM Person
WHERE Person.age >= 18
```

Since `age` is an optional `Person` field, we use `just` to lift
`val 18 :: SqlExpr (Value Int)` into `just (val 18) ::
SqlExpr (Value (Maybe Int))`.

## Joins

Implicit joins are represented by tuples.

For example, to get the list of all blog posts and their authors, we could
write:

```haskell
select $
from $ \(b, p) -> do
where_ (b ^. BlogPostAuthorId ==. p ^. PersonId)
orderBy [asc (b ^. BlogPostTitle)]
return (b, p)
```

which generates this SQL:

```sql
SELECT BlogPost.*, Person.*
FROM BlogPost, Person
WHERE BlogPost.authorId = Person.id
ORDER BY BlogPost.title ASC
```


However, you may want your results to include people who don't
have any blog posts as well using a `LEFT OUTER JOIN`:

```haskell
select $
from $ \(p `LeftOuterJoin`` mb) -> do
on (just (p ^. PersonId) ==. mb ?. BlogPostAuthorId)
orderBy [asc (p ^. PersonName), asc (mb ?. BlogPostTitle)]
return (p, mb)
```

which generates this SQL:

```sql
SELECT Person.*, BlogPost.*
FROM Person LEFT OUTER JOIN BlogPost
ON Person.id = BlogPost.authorId
ORDER BY Person.name ASC, BlogPost.title ASC
```

## Left Outer Join

On a `LEFT OUTER JOIN` the entity on the right hand side may
not exist (i.e. there may be a `Person` without any
`BlogPost`s), so while `p :: SqlExpr (Entity Person)`, we have
`mb :: SqlExpr (Maybe (Entity BlogPost))`.  The whole
expression above has type `SqlPersist m [(Entity Person, Maybe
(Entity BlogPost))]`.  Instead of using `(^.)`, we used
`(?.)` to project a field from a `Maybe (Entity a)`.

We are by no means limited to joins of two tables, nor by
joins of different tables.  For example, we may want a list
of the `Follow` entity:

```haskell
select $
from $ \(p1 `InnerJoin` f `InnerJoin` p2) -> do
on (p2 ^. PersonId ==. f ^. FollowFollowed)
on (p1 ^. PersonId ==. f ^. FollowFollower)
return (p1, f, p2)
```

which generates this SQL:

```sql
SELECT P1.*, Follow.*, P2.*
FROM Person AS P1
INNER JOIN Follow ON P1.id = Follow.follower
INNER JOIN P2 ON P2.id = Follow.followed
```

Note carefully that the order of the ON clauses is
reversed! You're required to write your `on`s in reverse
order because that helps composability (see the documentation
of `on` for more details).

## Update and Delete

```haskell
do update $ \p -> do
     set p [ PersonName =. val "João" ]
     where_ (p ^. PersonName ==. val "Joao")
   delete $
     from $ \p -> do
     where_ (p ^. PersonAge <. just (val 14))
```

The results of queries can also be used for insertions.
In `SQL`, we might write the following, inserting a new blog
post for every user:

```haskell
 insertSelect $ from $ \p->
 return $ BlogPost <# "Group Blog Post" <&> (p ^. PersonId)
```

which generates this SQL:

```sql
INSERT INTO BlogPost
SELECT ('Group Blog Post', id)
FROM Person
```

Individual insertions can be performed through Persistent's
`insert` function, reexported for convenience.

### Re-exports

We re-export many symbols from `persistent` for convenience:
- "Store functions" from "Database.Persist".
- Everything from "Database.Persist.Class" except for
 `PersistQuery` and `delete` (use `deleteKey` instead).
- Everything from "Database.Persist.Types" except for
 `Update`, `SelectOpt`, `BackendSpecificFilter` and `Filter`.
- Everything from "Database.Persist.Sql" except for
 `deleteWhereCount` and `updateWhereCount`.

### RDBMS Specific

There are many differences between SQL syntax and functions
supported by different RDBMSs.  Since version 2.2.8,
`esqueleto` includes modules containing functions that are
specific to a given RDBMS.

- PostgreSQL: `Database.Esqueleto.PostgreSQL`

In order to use these functions, you need to explicitly import
their corresponding modules