diff --git a/models.py b/models.py
new file mode 100644
index 00000000..10615d42
--- /dev/null
+++ b/models.py
@@ -0,0 +1,775 @@
+from collections import defaultdict
+import datetime
+import itertools
+
+from dateutil.parser import parse
+from dateutil.relativedelta import relativedelta
+from debts import settle
+from flask import current_app, g
+from flask_sqlalchemy import BaseQuery, SQLAlchemy
+from itsdangerous import (
+    BadSignature,
+    SignatureExpired,
+    URLSafeSerializer,
+    URLSafeTimedSerializer,
+)
+import sqlalchemy
+from sqlalchemy import orm
+from sqlalchemy.sql import func
+from sqlalchemy_continuum import make_versioned, version_class
+from sqlalchemy_continuum.plugins import FlaskPlugin
+from werkzeug.security import generate_password_hash
+
+from ihatemoney.currency_convertor import CurrencyConverter
+from ihatemoney.utils import get_members, same_bill
+from ihatemoney.versioning import (
+    ConditionalVersioningManager,
+    LoggingMode,
+    get_ip_if_allowed,
+    version_privacy_predicate,
+)
+
+make_versioned(
+    user_cls=None,
+    manager=ConditionalVersioningManager(
+        # Conditionally Disable the versioning based on each
+        # project's privacy preferences
+        tracking_predicate=version_privacy_predicate,
+    ),
+    plugins=[
+        FlaskPlugin(
+            # Redirect to our own function, which respects user preferences
+            # on IP address collection
+            remote_addr_factory=get_ip_if_allowed,
+            # Suppress the plugin's attempt to grab a user id,
+            # which imports the flask_login module (causing an error)
+            current_user_id_factory=lambda: None,
+        )
+    ],
+)
+
+db = SQLAlchemy()
+
+
+class Project(db.Model):
+    class ProjectQuery(BaseQuery):
+        def get_by_name(self, name):
+            return Project.query.filter(Project.name == name).one()
+
+    # Direct SQLAlchemy-Continuum to track changes to this model
+    __versioned__ = {}
+
+    id = db.Column(db.String(64), primary_key=True)
+
+    name = db.Column(db.UnicodeText)
+    password = db.Column(db.String(128))
+    contact_email = db.Column(db.String(128))
+    logging_preference = db.Column(
+        db.Enum(LoggingMode),
+        default=LoggingMode.default(),
+        nullable=False,
+        server_default=LoggingMode.default().name,
+    )
+    members = db.relationship("Person", backref="project")
+
+    query_class = ProjectQuery
+    default_currency = db.Column(db.String(3))
+
+    @property
+    def _to_serialize(self):
+        obj = {
+            "id": self.id,
+            "name": self.name,
+            "contact_email": self.contact_email,
+            "logging_preference": self.logging_preference.value,
+            "members": [],
+            "default_currency": self.default_currency,
+        }
+
+        balance = self.balance
+        for member in self.members:
+            member_obj = member._to_serialize
+            member_obj["balance"] = balance.get(member.id, 0)
+            obj["members"].append(member_obj)
+
+        return obj
+
+    @property
+    def active_members(self):
+        return [m for m in self.members if m.activated]
+
+    @property
+    def full_balance(self):
+        """Returns a triple of dicts:
+
+        - dict mapping each member to its balance
+
+        - dict mapping each member to how much he/she should pay others
+          (i.e. how much he/she benefited from bills)
+
+        - dict mapping each member to how much he/she should be paid by
+          others (i.e. how much he/she has paid for bills)
+
+        """
+        balances, should_pay, should_receive = (defaultdict(int) for time in (1, 2, 3))
+
+        for bill in self.get_bills_unordered().all():
+            should_receive[bill.payer.id] += bill.converted_amount
+            total_weight = sum(ower.weight for ower in bill.owers)
+            for ower in bill.owers:
+                should_pay[ower.id] += (
+                    ower.weight * bill.converted_amount / total_weight
+                )
+
+        for person in self.members:
+            balance = should_receive[person.id] - should_pay[person.id]
+            balances[person.id] = balance
+
+        return balances, should_pay, should_receive
+
+    @property
+    def balance(self):
+        return self.full_balance[0]
+
+    @property
+    def members_stats(self):
+        """Compute what each participant has paid
+
+        :return: one stat dict per participant
+        :rtype list:
+        """
+        balance, spent, paid = self.full_balance
+        return [
+            {
+                "member": member,
+                "paid": paid[member.id],
+                "spent": spent[member.id],
+                "balance": balance[member.id],
+            }
+            for member in self.active_members
+        ]
+
+    @property
+    def monthly_stats(self):
+        """Compute expenses by month
+
+        :return: a dict of years mapping to a dict of months mapping to the amount
+        :rtype dict:
+        """
+        monthly = defaultdict(lambda: defaultdict(float))
+        for bill in self.get_bills_unordered().all():
+            monthly[bill.date.year][bill.date.month] += bill.converted_amount
+        return monthly
+
+    @property
+    def uses_weights(self):
+        return len([i for i in self.members if i.weight != 1]) > 0
+
+    def get_transactions_to_settle_bill(self, pretty_output=False):
+        """Return a list of transactions that could be made to settle the bill"""
+
+        def prettify(transactions, pretty_output):
+            """Return pretty transactions"""
+            if not pretty_output:
+                return transactions
+            pretty_transactions = []
+            for transaction in transactions:
+                pretty_transactions.append(
+                    {
+                        "ower": transaction["ower"].name,
+                        "receiver": transaction["receiver"].name,
+                        "amount": round(transaction["amount"], 2),
+                        "currency": transaction["currency"],
+                    }
+                )
+            return pretty_transactions
+
+        # cache value for better performance
+        members = {person.id: person for person in self.members}
+        settle_plan = settle(self.balance.items()) or []
+
+        transactions = [
+            {
+                "ower": members[ower_id],
+                "receiver": members[receiver_id],
+                "amount": amount,
+                "currency": self.default_currency,
+            }
+            for ower_id, amount, receiver_id in settle_plan
+        ]
+
+        return prettify(transactions, pretty_output)
+
+    def exactmatch(self, credit, debts):
+        """Recursively try and find subsets of 'debts' whose sum is equal to credit"""
+        if not debts:
+            return None
+        if debts[0]["balance"] > credit:
+            return self.exactmatch(credit, debts[1:])
+        elif debts[0]["balance"] == credit:
+            return [debts[0]]
+        else:
+            match = self.exactmatch(credit - debts[0]["balance"], debts[1:])
+            if match:
+                match.append(debts[0])
+            else:
+                match = self.exactmatch(credit, debts[1:])
+            return match
+
+    def has_bills(self):
+        """return if the project do have bills or not"""
+        return self.get_bills_unordered().count() > 0
+
+    def has_multiple_currencies(self):
+        """Returns True if multiple currencies are used"""
+        # It would be more efficient to do the counting in the database,
+        # but this is called very rarely so we can tolerate if it's a bit
+        # slow. And doing this in Python is much more readable, see #784.
+        nb_currencies = len(
+            set(bill.original_currency for bill in self.get_bills_unordered())
+        )
+        return nb_currencies > 1
+
+    def get_bills_unordered(self):
+        """Base query for bill list"""
+        # The subqueryload option allows to pre-load data from the
+        # billowers table, which makes access to this data much faster.
+        # Without this option, any access to bill.owers would trigger a
+        # new SQL query, ruining overall performance.
+        return (
+            Bill.query.options(orm.subqueryload(Bill.owers))
+            .join(Person, Project)
+            .filter(Bill.payer_id == Person.id)
+            .filter(Person.project_id == Project.id)
+            .filter(Project.id == self.id)
+        )
+
+    def get_bills(self):
+        """Return the list of bills related to this project"""
+        return self.order_bills(self.get_bills_unordered())
+
+    @staticmethod
+    def order_bills(query):
+        return (
+            query.order_by(Bill.date.desc())
+            .order_by(Bill.creation_date.desc())
+            .order_by(Bill.id.desc())
+        )
+
+    def get_bill_weights(self):
+        """
+        Return all bills for this project, along with the sum of weight for each bill.
+        Each line is a (float, Bill) tuple.
+
+        Result is unordered.
+        """
+        return (
+            db.session.query(func.sum(Person.weight), Bill)
+            .options(orm.subqueryload(Bill.owers))
+            .select_from(Person)
+            .join(billowers, Bill, Project)
+            .filter(Person.project_id == Project.id)
+            .filter(Project.id == self.id)
+            .group_by(Bill.id)
+        )
+
+    def get_bill_weights_ordered(self):
+        """Ordered version of get_bill_weights"""
+        return self.order_bills(self.get_bill_weights())
+
+    def get_member_bills(self, member_id):
+        """Return the list of bills related to a specific member"""
+        return (
+            self.get_bills_unordered()
+            .filter(Person.id == member_id)
+            .order_by(Bill.date.desc())
+            .order_by(Bill.id.desc())
+        )
+
+    def get_newest_bill(self):
+        """Returns the most recent bill (according to bill date) or None if there are no bills"""
+        # Note that the ORM performs an optimized query with LIMIT
+        return self.get_bills_unordered().order_by(Bill.date.desc()).first()
+
+    def get_oldest_bill(self):
+        """Returns the least recent bill (according to bill date) or None if there are no bills"""
+        # Note that the ORM performs an optimized query with LIMIT
+        return self.get_bills_unordered().order_by(Bill.date.asc()).first()
+
+    def active_months_range(self):
+        """Returns a list of dates, representing the range of consecutive months
+        for which the project was active (i.e. has bills).
+
+        Note that the list might contain months during which there was no
+        bills.  We only guarantee that there were bills during the first
+        and last month in the list.
+        """
+        oldest_bill = self.get_oldest_bill()
+        newest_bill = self.get_newest_bill()
+        if oldest_bill is None or newest_bill is None:
+            return []
+        oldest_date = oldest_bill.date
+        newest_date = newest_bill.date
+        newest_month = datetime.date(
+            year=newest_date.year, month=newest_date.month, day=1
+        )
+        # Infinite iterator towards the past
+        all_months = (newest_month - relativedelta(months=i) for i in itertools.count())
+        # Stop when reaching one month before the first date
+        months = itertools.takewhile(
+            lambda x: x > oldest_date - relativedelta(months=1), all_months
+        )
+        return list(months)
+
+    def get_pretty_bills(self, export_format="json"):
+        """Return a list of project's bills with pretty formatting"""
+        bills = self.get_bills()
+        pretty_bills = []
+        for bill in bills:
+            if export_format == "json":
+                owers = [ower.name for ower in bill.owers]
+            else:
+                owers = ", ".join([ower.name for ower in bill.owers])
+
+            pretty_bills.append(
+                {
+                    "what": bill.what,
+                    "amount": round(bill.amount, 2),
+                    "currency": bill.original_currency,
+                    "date": str(bill.date),
+                    "payer_name": Person.query.get(bill.payer_id).name,
+                    "payer_weight": Person.query.get(bill.payer_id).weight,
+                    "owers": owers,
+                }
+            )
+        return pretty_bills
+
+    def switch_currency(self, new_currency):
+        if new_currency == self.default_currency:
+            return
+        # Update converted currency
+        if new_currency == CurrencyConverter.no_currency:
+            if self.has_multiple_currencies():
+                raise ValueError(f"Can't unset currency of project {self.id}")
+
+            for bill in self.get_bills_unordered():
+                # We are removing the currency, and we already checked that all bills
+                # had the same currency: it means that we can simply strip the currency
+                # without converting the amounts. We basically ignore the current default_currency
+
+                # Reset converted amount in case it was different from the original amount
+                bill.converted_amount = bill.amount
+                # Strip currency
+                bill.original_currency = CurrencyConverter.no_currency
+                db.session.add(bill)
+        else:
+            for bill in self.get_bills_unordered():
+                if bill.original_currency == CurrencyConverter.no_currency:
+                    # Bills that were created without currency will be set to the new currency
+                    bill.original_currency = new_currency
+                    bill.converted_amount = bill.amount
+                else:
+                    # Convert amount for others, without touching original_currency
+                    bill.converted_amount = CurrencyConverter().exchange_currency(
+                        bill.amount, bill.original_currency, new_currency
+                    )
+                db.session.add(bill)
+
+        self.default_currency = new_currency
+        db.session.add(self)
+        db.session.commit()
+
+    def import_bills(self, bills: list):
+        """Import bills from a list of dictionaries"""
+        # Add members not already in the project
+        project_members = [str(m) for m in self.members]
+        new_members = [
+            m for m in get_members(bills) if str(m[0]) not in project_members
+        ]
+        for m in new_members:
+            Person(name=m[0], project=self, weight=m[1])
+        db.session.commit()
+
+        # Import bills not already in the project
+        project_bills = self.get_pretty_bills()
+        id_dict = {m.name: m.id for m in self.members}
+        for b in bills:
+            same = False
+            for p_b in project_bills:
+                if same_bill(p_b, b):
+                    same = True
+                    break
+            if not same:
+                # Create bills
+                try:
+                    new_bill = Bill(
+                        amount=b["amount"],
+                        date=parse(b["date"]),
+                        external_link="",
+                        original_currency=b["currency"],
+                        owers=Person.query.get_by_names(b["owers"], self),
+                        payer_id=id_dict[b["payer_name"]],
+                        project_default_currency=self.default_currency,
+                        what=b["what"],
+                    )
+                except Exception as e:
+                    raise ValueError(f"Unable to import csv data: {repr(e)}")
+                db.session.add(new_bill)
+        db.session.commit()
+
+    def remove_member(self, member_id):
+        """Remove a member from the project.
+
+        If the member is not bound to a bill, then he is deleted, otherwise
+        he is only deactivated.
+
+        This method returns the status DELETED or DEACTIVATED regarding the
+        changes made.
+        """
+        person = Person.query.get(member_id, self)
+        if person is None:
+            return None
+        if not person.has_bills():
+            db.session.delete(person)
+            db.session.commit()
+        else:
+            person.activated = False
+            db.session.commit()
+        return person
+
+    def remove_project(self):
+        # We can't import at top level without circular dependencies
+        from ihatemoney.history import purge_history
+
+        db.session.delete(self)
+        # Purge AFTER delete to be sure to purge the deletion from history
+        purge_history(self)
+        db.session.commit()
+
+    def generate_token(self, token_type="auth"):
+        """Generate a timed and serialized JsonWebToken
+
+        :param token_type: Either "auth" for authentication (invalidated when project code changed),
+                        or "reset" for password reset (invalidated after expiration)
+        """
+
+        if token_type == "reset":
+            serializer = URLSafeTimedSerializer(
+                current_app.config["SECRET_KEY"], salt=token_type
+            )
+            token = serializer.dumps([self.id])
+        else:
+            serializer = URLSafeSerializer(
+                current_app.config["SECRET_KEY"] + self.password, salt=token_type
+            )
+            token = serializer.dumps([self.id])
+
+        return token
+
+    @staticmethod
+    def verify_token(token, token_type="auth", project_id=None, max_age=3600):
+        """Return the project id associated to the provided token,
+        None if the provided token is expired or not valid.
+
+        :param token: Serialized TimedJsonWebToken
+        :param token_type: Either "auth" for authentication (invalidated when project code changed),
+                        or "reset" for password reset (invalidated after expiration)
+        :param project_id: Project ID. Used for token_type "auth" to use the password as serializer
+                        secret key.
+        :param max_age: Token expiration time (in seconds). Only used with token_type "reset"
+        """
+        loads_kwargs = {}
+        if token_type == "reset":
+            serializer = URLSafeTimedSerializer(
+                current_app.config["SECRET_KEY"], salt=token_type
+            )
+            loads_kwargs["max_age"] = max_age
+        else:
+            project = Project.query.get(project_id) if project_id is not None else None
+            password = project.password if project is not None else ""
+            serializer = URLSafeSerializer(
+                current_app.config["SECRET_KEY"] + password, salt=token_type
+            )
+        try:
+            data = serializer.loads(token, **loads_kwargs)
+        except SignatureExpired:
+            return None
+        except BadSignature:
+            return None
+
+        data_project = data[0] if isinstance(data, list) else None
+        return (
+            data_project if project_id is None or data_project == project_id else None
+        )
+
+    def __str__(self):
+        return self.name
+
+    def __repr__(self):
+        return f"<Project {self.name}>"
+
+    @staticmethod
+    def create_demo_project():
+        project = Project(
+            id="demo",
+            name="demonstration",
+            password=generate_password_hash("demo"),
+            contact_email="demo@notmyidea.org",
+            default_currency="XXX",
+        )
+        db.session.add(project)
+        db.session.commit()
+
+        members = {}
+        for name in ("Amina", "Georg", "Alice"):
+            person = Person()
+            person.name = name
+            person.project = project
+            person.weight = 1
+            db.session.add(person)
+
+            members[name] = person
+
+        db.session.commit()
+
+        operations = (
+            ("Georg", 200, ("Amina", "Georg", "Alice"), "Food shopping"),
+            ("Alice", 20, ("Amina", "Alice"), "Beer !"),
+            ("Amina", 50, ("Amina", "Alice", "Georg"), "AMAP"),
+        )
+        for (payer, amount, owers, what) in operations:
+            db.session.add(
+                Bill(
+                    amount=amount,
+                    original_currency=project.default_currency,
+                    owers=[members[name] for name in owers],
+                    payer_id=members[payer].id,
+                    project_default_currency=project.default_currency,
+                    what=what,
+                )
+            )
+
+        db.session.commit()
+        return project
+
+
+class Person(db.Model):
+    class PersonQuery(BaseQuery):
+        def get_by_name(self, name, project):
+            return (
+                Person.query.filter(Person.name == name)
+                .filter(Person.project_id == project.id)
+                .one_or_none()
+            )
+
+        def get_by_names(self, names, project):
+            return (
+                Person.query.filter(Person.name.in_(names))
+                .filter(Person.project_id == project.id)
+                .all()
+            )
+
+        def get(self, id, project=None):
+            if not project:
+                project = g.project
+            return (
+                Person.query.filter(Person.id == id)
+                .filter(Person.project_id == project.id)
+                .one_or_none()
+            )
+
+        def get_by_ids(self, ids, project=None):
+            if not project:
+                project = g.project
+            return (
+                Person.query.filter(Person.id.in_(ids))
+                .filter(Person.project_id == project.id)
+                .all()
+            )
+
+    query_class = PersonQuery
+
+    # Direct SQLAlchemy-Continuum to track changes to this model
+    __versioned__ = {}
+
+    __table_args__ = {"sqlite_autoincrement": True}
+
+    id = db.Column(db.Integer, primary_key=True)
+    project_id = db.Column(db.String(64), db.ForeignKey("project.id"))
+    bills = db.relationship("Bill", backref="payer")
+
+    name = db.Column(db.UnicodeText)
+    weight = db.Column(db.Float, default=1)
+    activated = db.Column(db.Boolean, default=True)
+
+    @property
+    def _to_serialize(self):
+        return {
+            "id": self.id,
+            "name": self.name,
+            "weight": self.weight,
+            "activated": self.activated,
+        }
+
+    def has_bills(self):
+        """return if the participant do have bills or not"""
+        bills_as_ower_number = (
+            db.session.query(billowers)
+            .filter(billowers.columns.get("person_id") == self.id)
+            .count()
+        )
+        return bills_as_ower_number != 0 or len(self.bills) != 0
+
+    def __str__(self):
+        return self.name
+
+    def __repr__(self):
+        return f"<Person {self.name} for project {self.project.name}>"
+
+
+# We need to manually define a join table for m2m relations
+billowers = db.Table(
+    "billowers",
+    db.Column("bill_id", db.Integer, db.ForeignKey("bill.id"), primary_key=True),
+    db.Column("person_id", db.Integer, db.ForeignKey("person.id"), primary_key=True),
+    sqlite_autoincrement=True,
+)
+
+
+class Bill(db.Model):
+    class BillQuery(BaseQuery):
+        def get(self, project, id):
+            try:
+                return (
+                    self.join(Person, Project)
+                    .filter(Bill.payer_id == Person.id)
+                    .filter(Person.project_id == Project.id)
+                    .filter(Project.id == project.id)
+                    .filter(Bill.id == id)
+                    .one()
+                )
+            except orm.exc.NoResultFound:
+                return None
+
+        def delete(self, project, id):
+            bill = self.get(project, id)
+            if bill:
+                db.session.delete(bill)
+            return bill
+
+    query_class = BillQuery
+
+    # Direct SQLAlchemy-Continuum to track changes to this model
+    __versioned__ = {}
+
+    __table_args__ = {"sqlite_autoincrement": True}
+
+    id = db.Column(db.Integer, primary_key=True)
+
+    payer_id = db.Column(db.Integer, db.ForeignKey("person.id"))
+    owers = db.relationship(Person, secondary=billowers)
+
+    amount = db.Column(db.Float)
+    date = db.Column(db.Date, default=datetime.datetime.now)
+    creation_date = db.Column(db.Date, default=datetime.datetime.now)
+    what = db.Column(db.UnicodeText)
+    external_link = db.Column(db.UnicodeText)
+
+    original_currency = db.Column(db.String(3))
+    converted_amount = db.Column(db.Float)
+
+    archive = db.Column(db.Integer, db.ForeignKey("archive.id"))
+
+    currency_helper = CurrencyConverter()
+
+    def __init__(
+        self,
+        amount: float,
+        date: datetime.datetime = None,
+        external_link: str = "",
+        original_currency: str = "",
+        owers: list = [],
+        payer_id: int = None,
+        project_default_currency: str = "",
+        what: str = "",
+    ):
+        super().__init__()
+        self.amount = amount
+        self.date = date
+        self.external_link = external_link
+        self.original_currency = original_currency
+        self.owers = owers
+        self.payer_id = payer_id
+        self.what = what
+        self.converted_amount = self.currency_helper.exchange_currency(
+            self.amount, self.original_currency, project_default_currency
+        )
+
+    @property
+    def _to_serialize(self):
+        return {
+            "id": self.id,
+            "payer_id": self.payer_id,
+            "owers": self.owers,
+            "amount": self.amount,
+            "date": self.date,
+            "creation_date": self.creation_date,
+            "what": self.what,
+            "external_link": self.external_link,
+            "original_currency": self.original_currency,
+            "converted_amount": self.converted_amount,
+        }
+
+    def pay_each_default(self, amount):
+        """Compute what each share has to pay. Warning: this is slow, if you need
+        to compute this for many bills, do it differently (see
+        balance_full function)
+        """
+        if self.owers:
+            weights = (
+                db.session.query(func.sum(Person.weight))
+                .join(billowers, Bill)
+                .filter(Bill.id == self.id)
+            ).scalar()
+            return amount / weights
+        else:
+            return 0
+
+    def __str__(self):
+        return self.what
+
+    def pay_each(self):
+        """Warning: this is slow, if you need to compute this for many bills, do
+        it differently (see balance_full function)
+        """
+        return self.pay_each_default(self.converted_amount)
+
+    def __repr__(self):
+        return (
+            f"<Bill of {self.amount} from {self.payer} for "
+            f"{', '.join([o.name for o in self.owers])}>"
+        )
+
+
+class Archive(db.Model):
+    id = db.Column(db.Integer, primary_key=True)
+    project_id = db.Column(db.String(64), db.ForeignKey("project.id"))
+    name = db.Column(db.UnicodeText)
+
+    @property
+    def start_date(self):
+        pass
+
+    @property
+    def end_date(self):
+        pass
+
+    def __repr__(self):
+        return "<Archive>"
+
+
+sqlalchemy.orm.configure_mappers()
+
+PersonVersion = version_class(Person)
+ProjectVersion = version_class(Project)
+BillVersion = version_class(Bill)