pokedex/db/multilang.py

   1 from functools import partial
   2
   3 from sqlalchemy.ext.associationproxy import association_proxy
   4 from sqlalchemy.orm import 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
   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 TODO remove this requirement
  21
  22     Also supports the notion of a default language, which is attached to the
  23     session.  This is English by default, for historical and practical reasons.
  24
  25     Usage looks like this:
  26
  27         class Foo(Base): ...
  28
  29         create_translation_table('foo_bars', Foo, 'bars',
  30             name = Column(...),
  31         )
  32
  33         # Now you can do the following:
  34         foo.name
  35         foo.name_map['en']
  36         foo.foo_bars['en']
  37
  38         foo.name_map['en'] = "new name"
  39         del foo.name_map['en']
  40
  41         q.options(joinedload(Foo.bars_local))
  42         q.options(joinedload(Foo.bars))
  43
  44     The following properties are added to the passed class:
  45
  46     - `(relation_name)`, a relation to the new table.  It uses a dict-based
  47       collection class, where the keys are language identifiers and the values
  48       are rows in the created tables.
  49     - `(relation_name)_local`, a relation to the row in the new table that
  50       matches the current default language.
  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     # A foreign key "language_id" will clash with the language_id we naturally
  72     # put in every table.  Rename it something else
  73     if foreign_key_name == 'language_id':
  74         # TODO change language_id below instead and rename this
  75         foreign_key_name = 'lang_id'
  76
  77     Translations = type(_table_name, (object,), {
  78         '_language_identifier': association_proxy('language', 'identifier'),
  79     })
  80
  81     # Create the table object
  82     table = Table(_table_name, foreign_class.__table__.metadata,
  83         Column(foreign_key_name, Integer, ForeignKey(foreign_class.id),
  84             primary_key=True, nullable=False),
  85         Column('language_id', Integer, ForeignKey(language_class.id),
  86             primary_key=True, nullable=False),
  87     )
  88     Translations.__table__ = table
  89
  90     # Add ye columns
  91     # Column objects have a _creation_order attribute in ascending order; use
  92     # this to get the (unordered) kwargs sorted correctly
  93     kwitems = kwargs.items()
  94     kwitems.sort(key=lambda kv: kv[1]._creation_order)
  95     for name, column in kwitems:
  96         column.name = name
  97         table.append_column(column)
  98
  99     # Construct ye mapper
 100     mapper(Translations, table, properties={
 101         # TODO change to foreign_id
 102         'object_id': synonym(foreign_key_name),
 103         # TODO change this as appropriate
 104         'language': relationship(language_class,
 105             primaryjoin=table.c.language_id == language_class.id,
 106             lazy='joined',
 107             innerjoin=True),
 108         # TODO does this need to join to the original table?
 109     })
 110
 111     # Add full-table relations to the original class
 112     # Foo.bars
 113     setattr(foreign_class, relation_name, relationship(Translations,
 114         primaryjoin=foreign_class.id == Translations.object_id,
 115         collection_class=attribute_mapped_collection('language'),
 116         # TODO
 117         lazy='select',
 118     ))
 119     # Foo.bars_local
 120     # This is a bit clever; it uses bindparam() to make the join clause
 121     # modifiable on the fly.  db sessions know the current language identifier
 122     # populates the bindparam.
 123     local_relation_name = relation_name + '_local'
 124     setattr(foreign_class, local_relation_name, relationship(Translations,
 125         primaryjoin=and_(
 126             foreign_class.id == Translations.object_id,
 127             Translations._language_identifier ==
 128                 bindparam('_default_language', required=True),
 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.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
 158     Change the default_language attribute to whatever language's IDENTIFIER you
 159     would like to be the default.
 160     """
 161     default_language = 'en'
 162
 163     def execute(self, clause, params=None, *args, **kwargs):
 164         if not params:
 165             params = {}
 166         params.setdefault('_default_language', self.default_language)
 167         return super(MultilangSession, self).execute(
 168             clause, params, *args, **kwargs)