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# Relationships

Sqlkit makes editing of data that have relation as one2many or many2many very simple. It’s as easy as adding a field in the layout of the SqlMask:

    lay = """first_name last_name
             o2m=movies      - - -
           """

## One2Many in a SqlMak

This is a very powerful feature of `sqlkit` that is built on top of `sqlalchemy`‘s mechanism of properties and session and on the ability of `layout` to let you define in a symbolic way a widget.

In the following example we will use a `Mask` to represent Directors and a `Table` to represent films, connected to the first by a OneToMany relationship.

Suppose you already define a couple of tables in sqlalchemy, we will do it using elixir that slightly simplifies the syntax, in the package the demo uses declarative layer instead:

    class Director(elixir.Entity):
        last_name = elixir.Field(elixir.String(60))
        first_name = elixir.Field(elixir.String(60))
        movies = elixir.OneToMany('Movie', inverse='director', )
        elixir.using_options(tablename='director')

        def __repr__(self):
            return u'<Director: %s>' %self.last_name

    class Movie(elixir.Entity):
        title = elixir.Field(elixir.String(60))
        year = elixir.Field(elixir.Integer())
        director = elixir.ManyToOne('Director', inverse='movies' )
        elixir.using_options(tablename='movie')

        def __repr__(self):
            return u'<Movie: %s>' % self.title

To get a mask that:

> 1. lets you see director and films together
> 2. lets you edit them adding and deleting at will

You just have to start the SqlMask like this:

    lay = """first_name last_name
           o2m=movies      - - -
           """
    SqlMask(Director.mapper, metadata=__metadata__,
            session=elixir.session,layout=lay)

And you will get a mask that looks like this:

### delete behavior

We leave to sqlalchemy the responsibility to decide if a film must be deleted from the database or must just updated to set the director_id to NULL. To change this behavior you must act on the cascade property of the relation (or directly on the database definition). If yo want to delete a film in stead of just setting the property to NULL you can define the table in this way:

    class Director(elixir.Entity):
        ...
        movies = elixir.OneToMany('Movie', inverse='director',
                 cascade="all, delete-orphan" ### note this line
                        )
        ...

### Filtering

It is now possible to filter on a field present in related tables, e.g.: filtering all films that have a certain genre.

Note

Starting from 0.8.6-pre2 filters on a related table are aliased (see sqlalchemy docs). This is relevant if you filter on related table via different path (e.g: filtering staff and manager of a table projects, where both staff and managers are a m2m relation to User).

There are cases where this is really what you want but in some cases this is not. Here you really need to deeply understand what is happening underneath at the SQL level. As an example if you have a sqlmask of directors with a related table of films, if you filter on films and use a filter on title (containing ‘la’) and a filter on year (> 2005), the two filters will not be related to the same film: you’ll get all directors that have a film with ‘la’ in the title and (possibly a different) film produce after 2005. This may or may not be what you want...

## Many2many relationship

many2many relationship are as easy as one2many. Once again sqlalchemy does the heavy job. Definition is as simple as:

    lay = """year title
             m2m=genres - - -
             """
    tm = sqlkit.SqlMask(Movie.mapper, metadata=__metadata__,
                        session=elixir.session,layout=lay)
    lay = """
         name
         m2m=movies -
    """
    tm2 = sqlkit.SqlMask(Genre.mapper, metadata=__metadata__,
                         session=elixir.session,layout=lay)

and will pop up a couple of windows as the following, showing the same data from the two different main tables: movies with their genres and genres with they’re movies.

### adding & completion

Note

New in version 0.8.4.

When using Completion in a m2m table, adding from completion behaves differently that adding from m2o in that it requires the field to be already present and does not allow to edit it

This behavior can be changed setting it’s ‘m2m_editable’ property to True (new in 0.8.4):

    t.related.genres.set_editable(True)

## Many2One or ForeignKey

Many2One is a simpler case. The table we start from has a field that holds a ForeignKey, we just need to follow it to know the value. This again happens with no effort at all. In this case it’s also possible to use this field in a filter selection.

## Options

You can set the field_list directly from the layout as well as the number of rows:

    m2m=actors:5:first_name,last_name

will set a 5 rows table, and a field list of `first_name`, `last_name`. The real dimension of the table depends also on the expand attributes of the containers. you may need to set them to `gtk.EXPAND|gtk.FILL` by hand. There’s an example that demonstrates it.

## Behind the scenes

The way sqlkit understands that `movies` is an entry point for a relationship is that it analyzes the `mapper`, looks for a property with that name and realizes that it’s a PropertyLoader. That means that such an entry point has been put there by a `relation`.