models.py

from . import app, db


class Drink(db.Model):
    __tablename__ = 'drink'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), index=True, unique=True)
    recipe = db.relationship("DrinkIngredient", backref="drink")

    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return '<Drink %r>' % (self.name)


class DrinkIngredient(db.Model):
    __tablename__ = 'drink_ingredient'
    drink_id = db.Column(db.Integer, db.ForeignKey("drink.id"), primary_key=True)
    ingredient_id = db.Column(db.Integer, db.ForeignKey("ingredient.id"), primary_key=True)
    units = db.Column(db.Numeric(4, 2)) # Total of 4 digits, 2 after decimal: 00.00

    ingredient = db.relationship("Ingredient")

    def __init__(self, ingredient=None, units=None):
        self.ingredient = ingredient
        self.units = units

    def __repr__(self):
        return '<DrinkIngredient %g units of %r>' % (self.units, self.ingredient)


class Ingredient(db.Model):
    __tablename__ = 'ingredient'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(140))

    # subquery is also a valid option, if you want them loaded at query time. Select is the standard lazy loading
    # This needs to be viewonly, so new DrinkIngredients are spuriously created without units
    drinks = db.relationship("Drink", secondary=DrinkIngredient.__table__, lazy="select", viewonly=True)

    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return '<Ingredient %r>' % (self.name)

    @classmethod
    def get_or_create(cls, name):
        instance = cls.query.filter_by(name=name).first()
        if not instance:
            instance = cls(name)
            db.session.add(instance)
            db.session.flush([instance])
        return instance

usage.py

from models import Drink, DrinkIngredient, Ingredient

drink = Drink("Tequila Sunrise")
drink.recipe = [DrinkIngredient(Ingredient.get_or_create("Tequila"), 1),
                DrinkIngredient(Ingredient.get_or_create("Grenadine"), 0.2),
                DrinkIngredient(Ingredient.get_or_create("Orange Juice"), 4)]
db.session.add(drink)
db.session.commit()