Sometimes you want to be able to use synonyms in a SQLAlchemy mappers that point to columns that are present in the subclass rather than in the super class when using a polymorphic mapping. Unfortunately there is a bug in map_column=True and a workaround has to be used. The following workaround tells the mapper to map a column to a private variable and later adds the sysnonym to it.
Tables definition
In this example I will use the code I am currently working with, in this case the tables describe two different classes. One is the contact class which contains all the basic contact operations, while the second one is a person which describes the particular information stored about people and not other objects (like company).
self.__tables['contact'] = Table( DataManager.TablesNames['contact'], self.__metadata, Column('id', Integer, primary_key=True), Column('telephone_number', None, ForeignKey(\ DataManager.TablesNames["telephone_numbers"] + '.id'),nullable=True), Column('address', None, ForeignKey(DataManager.TablesNames['address'] + '.id'),nullable=True), Column('email_address', None, ForeignKey(DataManager.TablesNames['email_address'] + '.id'),nullable=True), Column('web', None, ForeignKey(DataManager.TablesNames['web_page'] + '.id'),nullable=True), Column('social_account', None, ForeignKey(DataManager.TablesNames['social_account'] + '.id'),nullable=True), Column('instant_messenger_account', None, ForeignKey(\ DataManager.TablesNames['instant_messenger'] + '.id'),nullable=True), Column('logo', Text, nullable=True, default=""), Column('comments', Text, nullable=True, default=""), Column('type', String(30), nullable=False)) self.__tables['contact_numbers'] = Table( DataManager.TablesNames['contact_number'], self.__metadata, Column('contact', None, ForeignKey(DataManager.TablesNames['contact'] + '.id'), primary_key=True), Column('number', None, ForeignKey( DataManager.TablesNames["telephone_numbers"] + '.id'), primary_key=True)) self.__tables['contact_addresses'] = Table( DataManager.TablesNames['contact_addresses'], self.__metadata, Column('contact', None, ForeignKey(\ DataManager.TablesNames['contact'] + '.id'), primary_key=True), Column('address', None, ForeignKey(\ DataManager.TablesNames['address'] + '.id'), primary_key=True)) self.__tables['contact_emails'] = Table( DataManager.TablesNames['contact_emails'], self.__metadata, Column('contact', None, ForeignKey(\ DataManager.TablesNames['contact'] + '.id'), primary_key=True), Column('email', None, ForeignKey(DataManager.TablesNames['email_address'] + '.id'), primary_key=True)) self.__tables['contact_webs'] = Table( DataManager.TablesNames['contact_webs'], self.__metadata, Column('contact', None, ForeignKey(\ DataManager.TablesNames['contact'] + '.id'), primary_key=True), Column('web', None, ForeignKey(\ DataManager.TablesNames['web_page'] + '.id'), primary_key=True)) self.__tables['contact_social_accounts'] = Table( DataManager.TablesNames['contact_social_accounts'], self.__metadata, Column('contact', None, ForeignKey(\ DataManager.TablesNames['contact'] + '.id'), primary_key=True), Column('account', None, ForeignKey(DataManager.TablesNames['social_account'] + '.id'), primary_key=True)) self.__tables['contact_ims'] = Table( DataManager.TablesNames['contact_ims'], self.__metadata, Column('contact', None, ForeignKey(\ DataManager.TablesNames['contact'] + '.id'), primary_key=True), Column('im', None, ForeignKey(\ DataManager.TablesNames['instant_messenger'] + '.id'), primary_key=True)) self.__tables['person'] = Table( DataManager.TablesNames['person'], self.__metadata, Column('contact_id', None, ForeignKey(DataManager.TablesNames['contact'] + '.id'), primary_key=True), Column('title', TitleEnum, nullable=True), Column('name', String(255), nullable=True, default="", index=True), Column('middle_name', String(255), nullable=True, default="", index=True), Column('surname', String(255), nullable=True, default="", index=True), Column('birthday', DateTime), Column('print_mode', PrintModeEnum, default=1) )
The mappings
The below mapping are just a simple set of mapping that allow to use a inheritance mapping between the contact table and the person table using a join.
mapper(Contact, self.__tables['contact'], polymorphic_on=self.__tables['contact'].c.type, polymorphic_identity='contact', with_polymorphic='*', properties={ 'logo' : synonym('_logo', map_column=True), 'numbers' : relation(TelephoneNumber, secondary=self.__tables['contact_numbers']), 'addresses' : relation(Address, secondary=self.__tables['contact_addresses']), 'emails' : relation(EmailAddress, secondary=self.__tables['contact_emails']), 'webs' : relation(Webpage, secondary=self.__tables['contact_webs']), 'social_accounts':relation(SocialAccount, secondary=self.__tables['contact_social_accounts']), 'instant_messengers':relation(InstantMessenger, secondary=self.__tables['contact_ims']), 'comments' : synonym('_comments', map_column=True), 'preferred_number' : relation(TelephoneNumber, uselist=False, primaryjoin=self.__tables['contact'].c.telephone_number==self.__tables['telephone_numbers'].c.id), 'preferred_address': relation(Address, uselist=False, primaryjoin=self.__tables['contact'].c.address==self.__tables['address'].c.id), 'preferred_email' : relation(EmailAddress, uselist=False, primaryjoin=self.__tables['contact'].c.email_address==self.__tables['email_address'].c.id), 'preferred_web' : relation(Webpage, uselist=False, primaryjoin=self.__tables['contact'].c.web==self.__tables['web_page'].c.id), 'preferred_social_account' : relation(SocialAccount, uselist=False, primaryjoin=self.__tables['contact'].c.social_account==self.__tables['social_account'].c.id), 'preferred_instant_messenger' : relation(InstantMessenger, uselist=False, primaryjoin=self.__tables['contact'].c.instant_messenger_account==self.__tables['instant_messenger'].c.id) }) mapper(Person, self.__tables['person'],inherits=Contact, polymorphic_identity='person', properties={ '_name': self.__tables['person'].c.name, 'name': synonym('_name'), '_middle_name': self.__tables['person'].c.middle_name, 'middle_name': synonym('_middle_name'), '_surname': self.__tables['person'].c.surname, 'surname': synonym('_surname'), '_title': self.__tables['person'].c.title, 'title': synonym('_title'), '_birthday': self.__tables['person'].c.birthday, 'birthday': synonym('_birthday'), '_print_mode': self.__tables['person'].c.print_mode, 'print_mode': synonym('_print_mode') })
In the above example we can see how we are mapping the column in the person mapper. The first step:
'_name': self.__tables['person'].c.name
maps the column with the private variable of the class, the second part of the “declaration”:
'name': synonym('_name')
makes use of the synonym allowing the class to use the property you declared in your class. This took some time to find out and I hope the post is useful to others 
