pokedex/db/multilang.py

   1 from functools import partial
   2
   3 from sqlalchemy.ext.associationproxy import association_proxy
   4 from sqlalchemy.orm import aliased, compile_mappers, mapper, relationship, synonym
   5 from sqlalchemy.orm.collections import attribute_mapped_collection
   6 from sqlalchemy.orm.session import Session, object_session
   7 from sqlalchemy.schema import Column, ForeignKey, Table
   8 from sqlalchemy.sql.expression import and_, bindparam, select
   9 from sqlalchemy.types import Integer
  10
  11 def create_translation_table(_table_name, foreign_class, relation_name,
  12     language_class, **kwargs):
  13     """Creates a table that represents some kind of data attached to the given
  14     foreign class, but translated across several languages.  Returns the new
  15     table's mapped class.  It won't be declarative, but it will have a
  16     `__table__` attribute so you can retrieve the Table object.
  17
  18     `foreign_class` must have a `__singlename__`, currently only used to create
  19     the name of the foreign key column.
  20
  21     Also supports the notion of a default language, which is attached to the
  22     session.  This is English by default, for historical and practical reasons.
  23
  24     Usage looks like this:
  25
  26         class Foo(Base): ...
  27
  28         create_translation_table('foo_bars', Foo, 'bars',
  29             name = Column(...),
  30         )
  31
  32         # Now you can do the following:
  33         foo.name
  34         foo.name_map['en']
  35         foo.foo_bars['en']
  36
  37         foo.name_map['en'] = "new name"
  38         del foo.name_map['en']
  39
  40         q.options(joinedload(Foo.bars_local))
  41         q.options(joinedload(Foo.bars))
  42
  43     The following properties are added to the passed class:
  44
  45     - `(relation_name)`, a relation to the new table.  It uses a dict-based
  46       collection class, where the keys are language identifiers and the values
  47       are rows in the created tables.
  48     - `(relation_name)_local`, a relation to the row in the new table that
  49       matches the current default language.
  50     - `(relation_name)_class`, the class created by this function.
  51
  52     Note that these are distinct relations.  Even though the former necessarily
  53     includes the latter, SQLAlchemy doesn't treat them as linked; loading one
  54     will not load the other.  Modifying both within the same transaction has
  55     undefined behavior.
  56
  57     For each column provided, the following additional attributes are added to
  58     Foo:
  59
  60     - `(column)_map`, an association proxy onto `foo_bars`.
  61     - `(column)`, an association proxy onto `foo_bars_local`.
  62
  63     Pardon the naming disparity, but the grammar suffers otherwise.
  64
  65     Modifying these directly is not likely to be a good idea.
  66     """
  67     # n.b.: language_class only exists for the sake of tests, which sometimes
  68     # want to create tables entirely separate from the pokedex metadata
  69
  70     foreign_key_name = foreign_class.__singlename__ + '_id'
  71
  72     Translations = type(_table_name, (object,), {
  73         '_language_identifier': association_proxy('local_language', 'identifier'),
  74     })
  75
  76     # Create the table object
  77     table = Table(_table_name, foreign_class.__table__.metadata,
  78         Column(foreign_key_name, Integer, ForeignKey(foreign_class.id),
  79             primary_key=True, nullable=False),
  80         Column('local_language_id', Integer, ForeignKey(language_class.id),
  81             primary_key=True, nullable=False),
  82     )
  83     Translations.__table__ = table
  84
  85     # Add ye columns
  86     # Column objects have a _creation_order attribute in ascending order; use
  87     # this to get the (unordered) kwargs sorted correctly
  88     kwitems = kwargs.items()
  89     kwitems.sort(key=lambda kv: kv[1]._creation_order)
  90     for name, column in kwitems:
  91         column.name = name
  92         table.append_column(column)
  93
  94     # Construct ye mapper
  95     mapper(Translations, table, properties={
  96         'foreign_id': synonym(foreign_key_name),
  97         'local_language': relationship(language_class,
  98             primaryjoin=table.c.local_language_id == language_class.id,
  99             lazy='joined',
 100             innerjoin=True),
 101     })
 102
 103     # Add full-table relations to the original class
 104     # Foo.bars_table
 105     setattr(foreign_class, relation_name + '_table', Translations)
 106     # Foo.bars
 107     setattr(foreign_class, relation_name, relationship(Translations,
 108         primaryjoin=foreign_class.id == Translations.foreign_id,
 109         collection_class=attribute_mapped_collection('local_language'),
 110         # TODO
 111         lazy='select',
 112     ))
 113     # Foo.bars_local
 114     # This is a bit clever; it uses bindparam() to make the join clause
 115     # modifiable on the fly.  db sessions know the current language identifier
 116     # populates the bindparam.  The manual alias and join are (a) to make the
 117     # condition nice (sqla prefers an EXISTS) and to make the columns play nice
 118     # when foreign_class == language_class.
 119     local_relation_name = relation_name + '_local'
 120     language_class_a = aliased(language_class)
 121     setattr(foreign_class, local_relation_name, relationship(Translations,
 122         primaryjoin=and_(
 123             foreign_class.id == Translations.foreign_id,
 124             Translations.local_language_id == select(
 125                 [language_class_a.id],
 126                 language_class_a.identifier ==
 127                     bindparam('_default_language', required=True),
 128             ),
 129         ),
 130         uselist=False,
 131         # TODO MORESO HERE
 132         lazy='select',
 133     ))
 134
 135     # Add per-column proxies to the original class
 136     for name, column in kwitems:
 137         # Class.(column) -- accessor for the default language's value
 138         setattr(foreign_class, name,
 139             association_proxy(local_relation_name, name))
 140
 141         # Class.(column)_map -- accessor for the language dict
 142         # Need a custom creator since Translations doesn't have an init, and
 143         # these are passed as *args anyway
 144         def creator(language, value):
 145             row = Translations()
 146             row.local_language = language
 147             setattr(row, name, value)
 148             return row
 149         setattr(foreign_class, name + '_map',
 150             association_proxy(relation_name, name, creator=creator))
 151
 152     # Done
 153     return Translations
 154
 155 class MultilangSession(Session):
 156     """A tiny Session subclass that adds support for a default language."""
 157     default_language = 'en'
 158
 159     def execute(self, clause, params=None, *args, **kwargs):
 160         if not params:
 161             params = {}
 162         params.setdefault('_default_language', self.default_language)
 163         return super(MultilangSession, self).execute(
 164             clause, params, *args, **kwargs)