Retrieving column from a SQLalchemy relationship

问题

I'm working on some wxpython widgets that integrate SQLalchemy, CRUD stuff. I've got a wx.ComboBox that lists the rows of a table linked by a relationship.

class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    name = Column(String(250), nullable=False)

class Category(Base):
    __tablename__ = 'category'
    id = Column(Integer, primary_key=True)
    name = Column(String(250), nullable=False)

class Thing(Base):
    __tablename__ = 'thing'
    id = Column(Integer, primary_key=True)
    description = Column(String(500), nullable=False)
    user_id = Column(Integer, ForeignKey('user.id'), nullable=True)
    user = relationship(User, foreign_keys = [user_id])
    category_id = Column(Integer, ForeignKey('category.id'), nullable=True)
    category = relationship(Category, foreign_keys = [category_id])


class RelationBox(wx.ComboBox):
    def __init__(self, parent, column):
        wx.ComboBox.__init__(self, parent, style = wx.CB_READONLY)
        self.nullable = True # column.nullable
        self.linked_table = column.mapper.class_

        if self.nullable:
            self.SetItems([""])

        self.options = session.query(self.linked_table)

        session.commit() 

        for option in self.options:
            self.Append(option.__repr__(), option)

I've simplified the code and only extracted some of it to hopefully give you a clearer picture. I've implemented it like this:

categories = ["user", "category"]
category_sizer = wx.BoxSizer(wx.HORIZONTAL)
self.column_widgets = {}

for category in categories:
    box = wx.BoxSizer(wx.VERTICAL)
    box.Add(wx.StaticText(self, -1, category.capitalize()), 0, wx.ALIGN_CENTRE|wx.ALL, 5)
    self.column_widgets[category] = RelationBox(self, getattr(Thing, category))
    box.Add(self.column_widgets[category], 1, wx.ALIGN_CENTRE|wx.ALIGN_TOP|wx.ALL, 2)
    category_sizer.Add(box, 0, wx.ALIGN_CENTRE|wx.ALL, 5)

I want to get the Column that is linked to the relationship so I can set whether the widget has a blank option or not.

回答1:

You can get the column associated with a relationship by inspecting the .prop attribute:

>>> Thing.category.prop.local_columns
set([Column('category_id', Integer(), ForeignKey('category.id'), table=<thing>)])
>>> Thing.category.prop.remote_side
set([Column('id', Integer(), table=<category>, primary_key=True, nullable=False)]))

Because there are two sides to a foreign key, you need to be careful which one (local_columns or remote_side) you choose.

To then fetch the value from an instance, do the following:

col, = Thing.category.prop.local_columns
key = Thing.__mapper__.get_property_by_column(col).key
getattr(thing, key)

来源：https://stackoverflow.com/questions/37566707/retrieving-column-from-a-sqlalchemy-relationship

标签

python

sqlalchemy

foreign-keys

relationship