db.relationship

• Allows SQLAlchemy to identify relationships between models
• Links relationships with backrefs (child1.some_parent)
• Configures relationship dynamics between parents and children
• db.relationship does not set up foreign key constraints for you. We need to add a column, some_parent_id, on the child model that has a foreign key constraint
• Whereas we set db.relationship on the parent mode, we set the foreign key constraint on the child model.
• A foreign key constraint prefers referential integrity from one table to another, by ensuring that the foreign key column always maps a primary key in the foreign table.

CREATE TABLE vehicles (
	id INTEGER PRIMARY KEY,
	make VARCHAR NOT NULL,
	model VARCHAR NOT NULL,
	year INTEGER NOT NULL,
	driver_id REFERENCES drivers(id)
)

driver_id maps to an id that belongs in the drivers table.
driver_id is nonempty

db.ForeignKey

• Option in db.column to specify a foreign key constraint, referring to the primary key of the other table / model
• Gets defined on the Child model

class SomeParent(db.Model):
	__tablename__ = 'some_parents'
	id = db.Column(db.Integer, primary_key=True)
	...

class SomeChild(db.Model):
	id = db.Column(db.Integer, primary_key=True)
	name = db.Column(db.String(50), nullable=False)
	#some_parents.id: Maps to parent table's primary key
	# format: {parent tablename}.{parent primary key column name}
	some_parent_id = db.Column(db.Integer, db.ForeignKey('some_parents.id'), nullable=False)

SQLAlchemy foreign keys Docs