sqlalchemy.exc.CircularDependencyError: Circular dependency detected

Question

The business logic - One Category may have multiple (1:M) attributes, like Category "Memory" could have attributes Speed, Size, Type etc.

at the same time one Category could be sorted by the attribute value (this is stored inside Category.sortByAttribute - which is foreign key to LookupCategoryAttributes table.

Trying to construct it via SQLAlchemy, but getting circular dependency detected. What is wrong?

class Attribute(Base):

    __tablename__ = "LookupCategoryAttributes"

    types = ["date", "float", "integer", "select", "string", "text"]

    # Properties
    ID                       = Column(BigInteger,    primary_key=True)
    categoryID               = Column(BigInteger,    ForeignKey('LookupCategories.ID'), nullable=False )
    attribute                = Column(VARCHAR(255),  nullable=False)
    listValues               = Column(VARCHAR(4000))
    typeID                   = Column(VARCHAR(40),   nullable=False)
    isRequired               = Column(SmallInteger,  nullable=False, default=0)
    displayInMenu            = Column(SmallInteger,  nullable=False, default=0)
    displayInFilter          = Column(SmallInteger,  nullable=False, default=0)


class Category(Base):

    __tablename__ = "LookupCategories"

    # Properties
    ID                       = Column(BigInteger,    primary_key=True)
    category                 = Column(VARCHAR(255),  nullable=False)
    description              = Column(VARCHAR(1000), nullable=False)
    parentCategoryID         = Column(BigInteger,    ForeignKey('LookupCategories.ID'))
    leftPos                  = Column(Integer)
    rightPos                 = Column(Integer)
    sortByAttribute          = Column(BigInteger,    ForeignKey('LookupCategoryAttributes.ID'))
    sortOrder                = Column(SmallInteger,  default=1)


    # Relationships
    ParentCategory    = relationship("Category",  uselist=False, remote_side=[ID], backref='SubCategories')
    SortByAttribute   = relationship("Attribute", uselist=False, foreign_keys=[sortByAttribute], primaryjoin="Attribute.ID==Category.sortByAttribute")
    Attributes        = relationship("Attribute", backref="Category", primaryjoin="Attribute.categoryID==Category.ID")

and then the code looks like this:

category = Category(record['Name'], extID=extID)
attr1 = Attribute(v)
attr2 = Attribute(v)

category.Attributes.append(attr1)
category.Attributes.append(attr2)
category.SortByAttribute = attr1

when I execute commit I get:

sqlalchemy.exc.CircularDependencyError: Circular dependency detected.

Answer 1

Okay found the answer - use post_update in relationship http://docs.sqlalchemy.org/en/latest/orm/relationship_persistence.html#post-update

so what I did is inside Category class is changed this:

SortByAttribute = relationship(
    "Attribute",
    uselist=False,
    foreign_keys=[sortByAttribute],
    primaryjoin="Attribute.ID==Category.sortByAttribute"
)

to this:

SortByAttribute = relationship(
    "Attribute",
    uselist=False,
    foreign_keys=[sortByAttribute],
    primaryjoin="Attribute.ID==Category.sortByAttribute",
    post_update=True
)