SQLAlchemy - Mapping self-referential relationship as one to many (declarative form)

You add a ForeignKey referencing the parent, and then create a relationship that specifies the direction via remote_side. This is documented under adjacency list relationships. For declarative you'd do something like this:

class Tag(Base):
    __tablename__ = 'tag'

    id = Column(Integer, primary_key=True)
    label = Column(String)
    parent_id = Column(Integer, ForeignKey('tag.id'))

    parent = relationship('Tag', remote_side=[id])

If you want the reverse relation also, add backref='children' to the relationship definition.

If you need children, you need to use uselist:

class Tag(Base):
    __tablename__ = 'tag'

    id = Column(Integer, primary_key=True)
    label = Column(String)
    child_id = Column(Integer, ForeignKey('tag.id'))

    children = relation('Tag', remote_side=[id], uselist=True)

class Company(BaseModel):
    __tablename__ = 'companies'
    companyName = db.Column(db.String(50))
    contactPerson = db.Column(db.String(50))
    email = db.Column(db.String(50))
    mobile = db.Column(db.String(20))
    parentID = db.Column(db.Integer, db.ForeignKey('companies.id'))  # parent company ID
    childrenCompany = db.relationship('Company', remote_side='Company.id',
                                    backref=db.backref('children_company'))  # parent Company

use:

In [2]: company_query = Company.query.get_or_404(1)

In [3]: company_query.children_company
Out[3]: 
[<app.models.user.Company at 0x10f527850>,
 <app.models.user.Company at 0x10f527c10>]