Question

My problem is as follows. I ve got a directed graph structure in sqlalchemy with polymorphic vertices and I d like to restrict the edge creation type dependant. The following code defines the graph structure.

from sqlalchemy import Column, ForeignKey, Integer
from sqlalchemy.orm import backref, relationship
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Edge(Base):
    __tablename__ =  edge 

    head_id = Column(Integer, ForeignKey( vertex.id ), primary_key=True)
    tail_id = Column(Integer, ForeignKey( vertex.id ), primary_key=True)

class Vertex(Base):
    __tablename__ =  vertex 

    id = Column( Integer, primary_key=True )
    predecessors = relationship( Vertex , 
                        secondary= edge ,
                        primaryjoin="Vertex.id==Edge.head_id",
                        secondaryjoin="Vertex.id==Edge.tail_id",
                        backref= successors )
    type    = Column( String(50) )

    __mapper_args__ = {
         polymorphic_identity : Vertex ,
         polymorphic_on :type
    }

图表的构造非常简单

#creates two vertices with a directed edge from v1 to v2
v1 = Vertex()
v2 = Vertex()
v1.successors.append(v2) # v1->v2

The problem is How can i restrict the edge creation type dependant. E.g. Edges from class A to class B are allowed but not vice versa.

class A(Vertex):
    pass

class B(Vertex):
    pass

v1 = A()
v2 = B()
v1.successors.append(v2) #allowed
v2.successors.append(v1) #forbidden

Answer 1

http://docs.sqlalchemy.org/en/rel_0_7/orm/mapper_config.html#simple-validators”rel=“nofollow” 简明标签:

from sqlalchemy.orm import validates

class Vertex(Base):
    # ...

    @validates( successors )
    def validate_successors(self, key, value):
        check_value = True
        # @todo: insert the validation logic here
        if type(self) == B and type(value) == A:
            check_value = False
        assert check_value,  Invalid Edge: not allowed to link from [{fr}] to [{to}] .format(fr=type(self).__name__, to=type(value).__name__)
        return value

<>strong>edit-1: 添加编码样本,将验证工作下放到衍生类别:

class Vertex(Base):
    # ...

    def _validate_successors_impl(self, value):
        return True

    @validates( successors )
    def validate_successors(self, key, value):
        self._validate_successors_impl(value)
        return value

class A(Vertex):
    __mapper_args__ = { polymorphic_identity :  A }

    def _validate_successors_impl(self, value):
        if type(value) == B:
            assert check_value,  Invalid Edge: not allowed to link from [{fr}] to [{to}] .format(fr=type(self).__name__, to=type(value).__name__)

class B(Vertex):
    __mapper_args__ = { polymorphic_identity :  B }

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