Database.Beam.Schema.Tables

Allows introspection into database types.

All database types must be of kind '(Type -> Type) -> Type'. If the type parameter is named f, each field must be of the type of f applied to some type for which an IsDatabaseEntity instance exists.

The be type parameter is necessary so that the compiler can ensure that backend-specific entities only work on the proper backend.

Entities are documented under the corresponding section and in the manual

Default derived function. Do not implement this yourself.

The idea is that, for any two databases over particular entity tags f and g, if we can take any entity in f and g to the corresponding entity in h (in the possibly effectful applicative functor m), then we can transform the two databases over f and g to a database in h, within m.

If that doesn't make sense, don't worry. This is mostly beam internal

When parameterized by this entity tag, a database type will hold meta-information on the Haskell mappings of database entities. Under the hood, each entity type is transformed into its DatabaseEntityDescriptor type. For tables this includes the table name as well as the corresponding TableSettings, which provides names for each column.

Represents a meta-description of a particular entityType. Mostly, a wrapper around 'DatabaseEntityDescriptor be entityType', but carries around the IsDatabaseEntity dictionary.

An entity tag for tables. See the documentation for Table or consult the manual for more.

A helper data type that lets you modify a database schema. Converts all entities in the database into functions from that entity to itself.

A newtype wrapper around 'f e -> f e' (i.e., an endomorphism between entity types in f). You usually want to use modifyTable or another function to contstruct these for you.

A newtype wrapper around 'Columnar f a -> Columnar f a' (i.e., an endomorphism between Columnars over f). You usually want to use fieldNamed or the IsString instance to rename the field, when 'f ~ TableField'

Return a DatabaseModification that does nothing. This is useful if you only want to rename one table. You can do

dbModification { tbl1 = modifyTable (\oldNm -> "NewTableName") tableModification }

Return a table modification (for use with modifyTable) that does nothing. Useful if you only want to change the table name, or if you only want to modify a few fields.

For example,

tableModification { field1 = "Column1" }

is a table modification (where 'f ~ TableField tbl') that changes the column name of field1 to Column1.

Modify a database according to a given modification. Most useful for DatabaseSettings to change the name mappings of tables and fields. For example, you can use this to modify the default names of a table

db :: DatabaseSettings MyDb
db = defaultDbSettings `withDbModification`
     dbModification {
       -- Change default name "table1" to "Table_1". Change the name of "table1Field1" to "first_name"
       table1 = setEntityName "Table_1" <> modifyTableFields tableModification { 
           table1Field1 = "first_name" 
        }
     }

Modify a table according to the given field modifications. Invoked by modifyTable to apply the modification in the database. Not used as often in user code, but provided for completeness.

Provide an EntityModification for TableEntitys. Allows you to modify the name of the table and provide a modification for each field in the table. See the examples for withDbModification for more.

Construct an EntityModification to rename any database entity

Change the entity name without consulting the beam-assigned one

Construct an EntityModification to rename the fields of a TableEntity

A field modification to rename the field. Also offered under the IsString instance for 'FieldModification (TableField tbl) a' for convenience.

Construct an EntityModification to set the schema of a database entity

Automatically provide names for tables, and descriptions for tables (using defTblFieldSettings). Your database must implement Generic, and must be auto-derivable. For more information on name generation, see the manual

Embed database settings in a larger database

A type family that we use to "tag" columns in our table datatypes.

This is what allows us to use the same table type to hold table data, describe table settings, derive lenses, and provide expressions.

The basic rules are

Columnar Identity x = x

Thus, any Beam table applied to Identity will yield a simplified version of the data type, that contains just what you'd expect.

The Nullable type is used when referencing PrimaryKeys that we want to include optionally. For example, if we have a table with a PrimaryKey, like the following

data BeamTableT f = BeamTableT
                  { _refToAnotherTable :: PrimaryKey AnotherTableT f
                  , ... }

we would typically be required to provide values for the PrimaryKey embedded into BeamTableT. We can use Nullable to lift this constraint.

data BeamTableT f = BeamTableT
                  { _refToAnotherTable :: PrimaryKey AnotherTableT (Nullable f)
                  , ... }

Now we can use just_ and nothing_ to refer to this table optionally. The embedded PrimaryKey in _refToAnotherTable automatically has its fields converted into Maybe using Nullable.

The last Columnar rule is

Columnar f x = f x

Use this rule if you'd like to parameterize your table type over any other functor. For example, this is used in the query modules to write expressions such as 'TableT QExpr', which returns a table whose fields have been turned into query expressions.

The other rules are used within Beam to provide lenses and to expose the inner structure of the data type.

A short type-alias for Columnar. May shorten your schema definitions

If you declare a function 'Columnar f a -> b' and try to constrain your function by a type class for f, GHC will complain, because f is ambiguous in 'Columnar f a'. For example, 'Columnar Identity (Maybe a) ~ Maybe a' and 'Columnar (Nullable Identity) a ~ Maybe a', so given a type 'Columnar f a', we cannot know the type of f.

Thus, if you need to know f, you can instead use Columnar'. Since its a newtype, it carries around the f paramater unambiguously. Internally, it simply wraps 'Columnar f a'

Like Data.Functor.Compose, but with an intermediate Columnar

Support for NULLable Foreign Key references.

data MyTable f = MyTable
               { nullableRef :: PrimaryKey AnotherTable (Nullable f)
               , ... }
                deriving (Generic, Typeable)

See Columnar for more information.

Metadata for a field of type ty in table.

Essentially a wrapper over the field name, but with a phantom type parameter, so that it forms an appropriate column tag.

Usually you use the defaultDbSettings function to generate an appropriate naming convention for you, and then modify it with withDbModification if necessary. Under this scheme, the field n be renamed using the IsString instance for TableField, or the fieldNamed function.

newtype mainly used to inspect the tag structure of a particular Beamable. Prevents overlapping instances in some case. Usually not used in end-user code.

Van Laarhoven lens to retrieve or set the field name from a TableField.

Represents a table that contains metadata on its fields. In particular, each field of type 'Columnar f a' is transformed into 'TableField table a'. You can get or update the name of each field by using the fieldName lens.

The regular Haskell version of the table. Equivalent to 'tbl Identity'

A form of table all fields Ignored. Useful as a parameter to zipTables when you only care about one table.

Column tag that ignores the type.

Carry a constraint instance and the value it applies to.

Carry a constraint instance.

Class to automatically unwrap nested Nullables

The big Kahuna! All beam tables implement this class.

The kind of all table types is '(Type -> Type) -> Type'. This is because all table types are actually table type constructors. Every table type takes in another type constructor, called the column tag, and uses that constructor to instantiate the column types. See the documentation for Columnar.

This class is mostly Generic-derivable. You need only specify a type for the table's primary key and a method to extract the primary key given the table.

An example table:

data BlogPostT f = BlogPost
                 { _blogPostSlug    :: Columnar f Text
                 , _blogPostBody    :: Columnar f Text
                 , _blogPostDate    :: Columnar f UTCTime
                 , _blogPostAuthor  :: PrimaryKey AuthorT f
                 , _blogPostTagline :: Columnar f (Maybe Text)
                 , _blogPostImageGallery :: PrimaryKey ImageGalleryT (Nullable f) }
                   deriving Generic
instance Beamable BlogPostT
instance Table BlogPostT where
   data PrimaryKey BlogPostT f = BlogPostId (Columnar f Text) deriving Generic
   primaryKey = BlogPostId . _blogPostSlug
instance Beamable (PrimaryKey BlogPostT)

We can interpret this as follows:

• The _blogPostSlug, _blogPostBody, _blogPostDate, and _blogPostTagline fields are of types Text, Text, UTCTime, and 'Maybe Text' respectfully.

• Since _blogPostSlug, _blogPostBody, _blogPostDate, _blogPostAuthor must be provided (i.e, they cannot contain Nothing), they will be given SQL NOT NULL constraints. _blogPostTagline is declared Maybe so Nothing will be stored as NULL in the database. _blogPostImageGallery will be allowed to be empty because it uses the Nullable tag modifier.

• blogPostAuthor references the AuthorT table (not given here) and is required.

• blogPostImageGallery references the ImageGalleryT table (not given here), but this relation is not required (i.e., it may be Nothing. See Nullable).

Provides a number of introspection routines for the beam library. Allows us to "zip" tables with different column tags together. Always instantiate an empty Beamable instance for tables, primary keys, and any type that you would like to embed within either. See the manual for more information on embedding.

Products: encode multiple arguments to constructors

Return a TableSettings for the appropriate table type where each column has been given its default name. See the manual for information on the default naming convention.

Synonym for primaryKey

Produce the beam default field name for the given path