SQLAlchemy: get Model from table name. This may im

问题:

I want to make a function that, given the name of a table, returns the model with that tablename. Eg:

class Model(Base):
    __tablename__ = 'table'
    ...a bunch of Columns

def getModelFromTableName(tablename):
   ...something magical

so getModelFromTableName('table') should return the Model class.

My aim is to use the function in a simple form generator I'm making since FormAlchemy does not work with python3.2 and I want it to handle foreign keys nicely.

Can anyone give me any pointers on how to get getModelFromTableName to work?

Here's one idea I have (it might be totally wrong, I haven't worked with meta classes before...)

What if I were to make my Model classes inherit from Base as well as some other class (TableReg) and have the class meta of TableReg store Model.tablename in some global dictionary or Singleton.

I realise this could be totally off because Base's metaclass does some very important and totally nifty stuff that I don't want to break, but I assume there has to be a way for me to append a little bit of constructor code to the meta class of my models. Or I don't understand.

回答1:

Inspired by Eevee's comment:

def get_class_by_tablename(tablename):
  """Return class reference mapped to table.

  :param tablename: String with name of table.
  :return: Class reference or None.
  """
  for c in Base._decl_class_registry.values():
    if hasattr(c, '__tablename__') and c.__tablename__ == tablename:
      return c

回答2:

Utility function for this has been added to SQLAlchemy-Utils. See get_class_by_table docs for more information. The solution in SQLAlchemy-Utils is able to cover single table inheritance scenarios as well.

import sqlalchemy as sa


def get_class_by_table(base, table, data=None):
    """
    Return declarative class associated with given table. If no class is found
    this function returns `None`. If multiple classes were found (polymorphic
    cases) additional `data` parameter can be given to hint which class
    to return.

    ::

        class User(Base):
            __tablename__ = 'entity'
            id = sa.Column(sa.Integer, primary_key=True)
            name = sa.Column(sa.String)


        get_class_by_table(Base, User.__table__)  # User class


    This function also supports models using single table inheritance.
    Additional data paratemer should be provided in these case.

    ::

        class Entity(Base):
            __tablename__ = 'entity'
            id = sa.Column(sa.Integer, primary_key=True)
            name = sa.Column(sa.String)
            type = sa.Column(sa.String)
            __mapper_args__ = {
                'polymorphic_on': type,
                'polymorphic_identity': 'entity'
            }

        class User(Entity):
            __mapper_args__ = {
                'polymorphic_identity': 'user'
            }


        # Entity class
        get_class_by_table(Base, Entity.__table__, {'type': 'entity'})

        # User class
        get_class_by_table(Base, Entity.__table__, {'type': 'user'})


    :param base: Declarative model base
    :param table: SQLAlchemy Table object
    :param data: Data row to determine the class in polymorphic scenarios
    :return: Declarative class or None.
    """
    found_classes = set(
        c for c in base._decl_class_registry.values()
        if hasattr(c, '__table__') and c.__table__ is table
    )
    if len(found_classes) > 1:
        if not data:
            raise ValueError(
                "Multiple declarative classes found for table '{0}'. "
                "Please provide data parameter for this function to be able "
                "to determine polymorphic scenarios.".format(
                    table.name
                )
            )
        else:
            for cls in found_classes:
                mapper = sa.inspect(cls)
                polymorphic_on = mapper.polymorphic_on.name
                if polymorphic_on in data:
                    if data[polymorphic_on] == mapper.polymorphic_identity:
                        return cls
            raise ValueError(
                "Multiple declarative classes found for table '{0}'. Given "
                "data row does not match any polymorphic identity of the "
                "found classes.".format(
                    table.name
                )
            )
    elif found_classes:
        return found_classes.pop()
    return None

回答3:

Beware the OrangeTux answer does not take schemas in account. If you have table homonyms in different schemas use:

def get_class_by_tablename(table_fullname):
  """Return class reference mapped to table.

  :param table_fullname: String with fullname of table.
  :return: Class reference or None.
  """
  for c in Base._decl_class_registry.values():
    if hasattr(c, '__table__') and c.__table__.fullname == table_fullname:
      return c

fullname is a Table attribute see: https://github.com/zzzeek/sqlalchemy/blob/master/lib/sqlalchemy/sql/schema.py#L455

回答4:

if you are working with sqlalchemy automap, you need a slight modification. Took me couple minutes to tune it out:

def get_class_by_tablename(tablename, Base):
  for c in Base._decl_class_registry.values():
    if c.__table__.name == tablename:
      return c

回答5:

I was going to delete this but I figure that the discussion in the comments might be useful for people who want to know about some good practices. Take this answer with a pinch of salt.

---

something like this does the trick:

def getModelFromTableName(sTable):
    """
    return the Model class with the given __tablename__
    """
    globals = globals()
    for k in globals:
        if type(globals[k]) == sqlalchemy.ext.declarative.DeclarativeMeta:
            try:
                if globals[k].__tablename__ == sTable:
                    return globals[k]
            except:
                pass
    return None