core.py

"""
Design Overview
---------------

The core models consist of:

- ``Account`` - Such as 'Accounts Receivable', a bank account, etc.
  Accounts can be arranged as a tree structure,
  where the balance of the parent account is the summation of the balances of all its children.
- ``Transaction`` - Represents a movement between accounts. Each transaction must have two or more legs.
- ``Leg`` - Represents a flow of money into (debit) or out of (credit) a transaction.
  Debits are represented by negative amounts, and credits by positive amounts.
  The sum of all a transaction's legs must equal zero.
  This is enforced with a database constraint.

Additionally, there are models which related to the import of external bank statement data:

- ``StatementImport`` - Represents a simple import of zero or more statement
  lines relating to a specific ``Account``.
- ``StatementLine`` - Represents a statement line. ``StatementLine.create_transaction()`` may be called to
  create a transaction for the statement line.
"""

from datetime import date

from django.db import connection, models
from django.db import transaction
from django.db import transaction as db_transaction
from django.db.models import F, JSONField
from django.utils import timezone
from django.utils.translation import gettext_lazy as _
from djmoney.models.fields import MoneyField
from djmoney.settings import CURRENCY_CHOICES
from moneyed import CurrencyDoesNotExist, Money
from mptt.models import MPTTModel, TreeForeignKey, TreeManager

from hordak import exceptions
from hordak.defaults import (
    DECIMAL_PLACES,
    DEFAULT_CURRENCY,
    MAX_DIGITS,
    UUID_DEFAULT,
    get_internal_currency,
)
from hordak.utilities.currency import Balance
from hordak.utilities.db_functions import GetBalance
from hordak.utilities.dreprecation import deprecated


#: Debit
DEBIT = "debit"
#: Credit
CREDIT = "credit"


def json_default():
    return {}


def get_currency_choices():
    return CURRENCY_CHOICES


class AccountQuerySet(models.QuerySet):
    def net_balance(self, raw=False):
        return sum((account.balance(raw) for account in self), Balance())

    def with_balances(self, as_of: date = None):
        """Annotate the account queryset with account balances

        This is a much more performant way to calculate account balances,
        especially when calculating balances for a lot of accounts.

        Note that you will get better performance by setting the `as_of`
        to `None`. This is because the underlying custom database function
        can avoid a join.
        """
        return self.annotate(balance=GetBalance(F("id"), as_of=as_of))


class AccountManager(TreeManager):
    def get_by_natural_key(self, uuid):
        return self.get(uuid=uuid)


class AccountType(models.TextChoices):
    # Eg. Cash in bank
    asset = "AS", _("Asset")
    # Eg. Loans, bills paid after the fact (in arrears)
    liability = "LI", _("Liability")
    # Eg. Sales, housemate contributions
    income = "IN", _("Income")
    # Eg. Office supplies, paying bills
    expense = "EX", _("Expense")
    # Eg. Money from shares
    equity = "EQ", _("Equity")
    # Used to represent currency conversions
    trading = "TR", _("Currency Trading")


def account_default_currencies():
    return (DEFAULT_CURRENCY,)


class Account(MPTTModel):
    """Represents an account

    An account may have a parent, and may have zero or more children. Only root
    accounts can have a type, all child accounts are assumed to have the same
    type as their parent.

    An account's balance is calculated as the sum of all of the transaction Leg's
    referencing the account.

    Attributes:

        uuid (UUID): UUID for account. Use to prevent leaking of IDs (if desired).
        name (str): Name of the account. Required.
        parent (Account|None): Parent account, nonen if root account
        code (str): Account code. Must combine with account codes of parent
            accounts to get fully qualified account code.
        type (str): Type of account as defined by `AccountType`. Can only be set on
            root accounts. Child accounts are assumed to have the same time as their parent.
        is_bank_account (bool): Is this a bank account. This implies we can import bank statements into
            it and that it only supports a single currency.


    """

    uuid = models.UUIDField(
        default=UUID_DEFAULT, editable=False, verbose_name=_("uuid")
    )
    name = models.CharField(max_length=255, verbose_name=_("name"))
    parent = TreeForeignKey(
        "self",
        null=True,
        blank=True,
        related_name="children",
        db_index=True,
        on_delete=models.CASCADE,
        verbose_name=_("parent"),
    )
    code = models.CharField(max_length=6, null=True, blank=True, verbose_name=_("code"))
    full_code = models.CharField(
        max_length=255,
        db_index=True,
        unique=True,
        null=True,
        blank=True,
        verbose_name=_("full_code"),
    )
    # TODO: Implement this child_code_width field, as it is probably a good idea
    # child_code_width = models.PositiveSmallIntegerField(default=1)
    type = models.CharField(
        max_length=2, choices=AccountType.choices, blank=True, verbose_name=_("type")
    )
    is_bank_account = models.BooleanField(
        default=False,
        blank=True,
        help_text="Is this a bank account. This implies we can import bank "
        "statements into it and that it only supports a single currency",
        verbose_name=_("is bank account"),
    )
    currencies = JSONField(
        db_index=True,
        default=account_default_currencies,
        verbose_name=_("currencies"),
    )

    objects = AccountManager.from_queryset(AccountQuerySet)()

    class MPTTMeta:
        order_insertion_by = ["code"]

    class Meta:
        unique_together = (("parent", "code"),)
        verbose_name = _("account")

    def __init__(self, *args, **kwargs):
        super(Account, self).__init__(*args, **kwargs)
        self._initial_code = self.code

    def save(self, *args, **kwargs):
        is_creating = not bool(self.pk)
        if is_creating:
            update_fields = None
        else:
            # See issues #19 & #31. It seems that on Django 1.2, django-mptt's left/right
            # tree fields get overwritten on save. The solution here is to exclude them from
            # being modified upon saving by using the save methods' update_fields argument.
            update_fields = [
                "uuid",
                "name",
                "parent",
                "code",
                "type",
                "is_bank_account",
                "currencies",
            ]
        super(Account, self).save(*args, update_fields=update_fields, **kwargs)

        if connection.vendor == "mysql":
            # We need updated lft/rght/tree_id values for the mysql_run_manual_trigger() call
            self.refresh_from_db()

        mysql_simulate_trigger(
            "update_full_account_codes", self.lft, self.rght, self.tree_id
        )

        do_refresh = False

        # If we've just created a non-root node then we're going to need to load
        # the type back from the DB (as it is set by trigger)
        if is_creating and not self.is_root_node():
            do_refresh = True

        # If we've just create this account or if the code has changed then we're
        # going to need to reload from the DB (full_code is set by trigger)
        if is_creating or self._initial_code != self.code:
            do_refresh = True

        if do_refresh:
            self.refresh_from_db()

    @classmethod
    def validate_accounting_equation(cls):
        """Check that all accounts sum to 0"""
        balances = [
            account.balance(raw=True) for account in Account.objects.root_nodes()
        ]
        if sum(balances, Balance()) != 0:
            raise exceptions.AccountingEquationViolationError(
                "Account balances do not sum to zero. They sum to {}".format(
                    sum(balances)
                )
            )

    def __str__(self):
        name = self.name or "Unnamed Account"
        if self.is_leaf_node():
            try:
                balance = self.balance()
            except (ValueError, CurrencyDoesNotExist):
                if self.full_code:
                    return "{} {}".format(self.full_code, name)
                else:
                    return name
            else:
                if self.full_code:
                    return "{} {} [{}]".format(self.full_code, name, balance)
                else:
                    return "{} [{}]".format(name, balance)

        else:
            return name

    def natural_key(self):
        return (self.uuid,)

    @property
    def sign(self):
        """
        Returns 1 if a credit should increase the value of the
        account, or -1 if a credit should decrease the value of the
        account.

        This is based on the account type as is standard accounting practice.
        The signs can be derrived from the following expanded form of the
        accounting equation:

            Assets = Liabilities + Equity + (Income - Expenses)

        Which can be rearranged as:

            0 = Liabilities + Equity + Income - Expenses - Assets

        Further details here: https://en.wikipedia.org/wiki/Debits_and_credits

        """
        return -1 if self.type in (AccountType.asset, AccountType.expense) else 1

    def balance(self, as_of=None, raw=False, leg_query=None, **kwargs):
        """Get the balance for this account, including child accounts

        Args:
            as_of (Date): Only include transactions on or before this date
            raw (bool): If true the returned balance should not have its sign
                        adjusted for display purposes.
            kwargs (dict): Will be used to filter the transaction legs

        Returns:
            Balance

        See Also:
            :meth:`simple_balance()`
        """
        balances = [
            account.simple_balance(as_of=as_of, raw=raw, leg_query=leg_query, **kwargs)
            for account in self.get_descendants(include_self=True)
        ]
        return sum(balances, Balance())

    def simple_balance(self, as_of=None, raw=False, leg_query=None, **kwargs):
        """Get the balance for this account, ignoring all child accounts

        Args:
            as_of (Date): Only include transactions on or before this date
            raw (bool): If true the returned balance should not have its sign
                        adjusted for display purposes.
            leg_query (models.Q): Django Q-expression, will be used to filter the transaction legs.
                                  allows for more complex filtering than that provided by **kwargs.
            kwargs (dict): Will be used to filter the transaction legs

        Returns:
            Balance
        """
        legs = self.legs
        if as_of:
            legs = legs.filter(transaction__date__lte=as_of)

        if leg_query or kwargs:
            leg_query = leg_query or models.Q()
            legs = legs.filter(leg_query, **kwargs)

        return legs.sum_to_balance() * (1 if raw else self.sign) + self._zero_balance()

    def _zero_balance(self):
        """Get a balance for this account with all currencies set to zero"""
        return Balance([Money("0", currency) for currency in self.currencies])

    @db_transaction.atomic()
    def transfer_to(self, to_account, amount, **transaction_kwargs):
        """Create a transaction which credits self and debits `to_account`.

        See https://en.wikipedia.org/wiki/Double-entry_bookkeeping.

        This is a shortcut utility method which simplifies the process of
        transferring where `self` is Cr and `to_account` is Dr.

        For example:

          * Transferring income -> income will result in the former increasing and the latter decreasing
          * Transferring income -> asset (i.e. bank) will result in the balance of both increasing
          * Transferring asset -> asset will result in the former decreasing and the latter increasing

        .. note::

                    LHS                         RHS
            ``{asset | expense} <-> {income | liability | equity}``

            Transfers LHS (A) -> RHS (B) will decrease A and increase B
            Transfers LHS (A) -> LHS (B) will decrease A and increase B
            Transfers RHS (A) -> LHS (B) will increase A and increase B
            Transfers RHS (A) -> RHS (B) will increase A and decrease B

        Args:

            to_account (Account): The destination account.
            amount (Money): The amount to be transferred.
            transaction_kwargs: Passed through to transaction creation. Useful for setting the
                transaction `description` field.
        """
        if not isinstance(amount, Money):
            raise TypeError("amount must be of type Money")

        transaction = Transaction.objects.create(**transaction_kwargs)

        Leg.objects.create(transaction=transaction, account=self, amount=+amount)
        Leg.objects.create(transaction=transaction, account=to_account, amount=-amount)

        return transaction

    @deprecated(
        "accounting_transfer_to() has been renamed to transfer_to(). Update your "
        "code to call transfer_to() directly. This will become an error in Hordak 3."
    )
    def accounting_transfer_to(self, *args, **kwargs):
        return self.transfer_to(*args, **kwargs)


class TransactionManager(models.Manager):
    def get_by_natural_key(self, uuid):
        return self.get(uuid=uuid)


class Transaction(models.Model):
    """Represents a transaction

    A transaction is a movement of funds between two accounts. Each transaction
    will have two or more legs, each leg specifies an account and an amount.

    .. note:

        When working with Hordak Transaction objects you will typically need to do so
        within a database transaction. This is because the database has integrity checks in
        place to ensure the validity of the transaction (i.e. money in = money out).

    See Also:

        :meth:`Account.transfer_to()` is a useful shortcut to avoid having to create transactions manually.

    Examples:

        You can manually create a transaction as follows::

            from django.db import transaction as db_transaction
            from hordak.models import Transaction, Leg

            with db_transaction.atomic():
                transaction = Transaction.objects.create()
                Leg.objects.create(transaction=transaction, account=my_account1, amount=Money(100, 'EUR'))
                Leg.objects.create(transaction=transaction, account=my_account2, amount=Money(-100, 'EUR'))

    Attributes:

        uuid (models.UUIDField): UUID for transaction. Use to prevent leaking of IDs (if desired).
        timestamp (datetime): The datetime when the object was created.
        date (date): The date when the transaction actually occurred, as this may be different to
            :attr:`timestamp`.
        description (str): Optional user-provided description

    """

    uuid = models.UUIDField(
        default=UUID_DEFAULT, editable=False, verbose_name=_("uuid")
    )
    timestamp = models.DateTimeField(
        default=timezone.now,
        help_text="The creation date of this transaction object",
        verbose_name=_("timestamp"),
    )
    date = models.DateField(
        default=timezone.now,
        help_text="The date on which this transaction occurred",
        verbose_name=_("date"),
    )
    description = models.TextField(
        default="", blank=True, verbose_name=_("description")
    )

    objects = TransactionManager()

    class Meta:
        get_latest_by = "date"
        verbose_name = _("transaction")

    def balance(self):
        return self.legs.sum_to_balance()

    def natural_key(self):
        return (self.uuid,)


class LegQuerySet(models.QuerySet):
    def sum_to_balance(self):
        """Sum the Legs of the QuerySet to get a `Balance`_ object"""
        result = self.values("amount_currency").annotate(total=models.Sum("amount"))
        return Balance([Money(r["total"], r["amount_currency"]) for r in result])


class LegManager(models.Manager):
    def get_by_natural_key(self, uuid):
        return self.get(uuid=uuid)

    def debits(self):
        """Filter for legs that were debits"""
        return self.filter(amount__gt=0)

    def credits(self):
        """Filter for legs that were credits"""
        return self.filter(amount__lt=0)


CustomLegManager = LegManager.from_queryset(LegQuerySet)


class Leg(models.Model):
    """The leg of a transaction

    Represents a single amount either into or out of a transaction. All legs for a transaction
    must sum to zero, all legs must be of the same currency.

    Attributes:

        uuid (UUID): UUID for transaction leg. Use to prevent leaking of IDs (if desired).
        transaction (Transaction): Transaction to which the Leg belongs.
        account (Account): Account the leg is transferring to/from.
        amount (Money): The amount being transferred
        description (str): Optional user-provided description
        type (str): :attr:`hordak.models.DEBIT` or :attr:`hordak.models.CREDIT`.

    """

    uuid = models.UUIDField(
        default=UUID_DEFAULT, editable=False, verbose_name=_("uuid")
    )
    transaction = models.ForeignKey(
        Transaction,
        related_name="legs",
        on_delete=models.CASCADE,
        verbose_name=_("transaction"),
    )
    account = models.ForeignKey(
        Account,
        related_name="legs",
        on_delete=models.CASCADE,
        verbose_name=_("account"),
    )
    amount = MoneyField(
        max_digits=MAX_DIGITS,
        decimal_places=DECIMAL_PLACES,
        help_text="Record debits as positive, credits as negative",
        default_currency=get_internal_currency,
        verbose_name=_("amount"),
    )
    description = models.TextField(
        default="", blank=True, verbose_name=_("description")
    )

    objects = CustomLegManager()

    def __str__(self):
        return f"{self.type.title()} {self.account.name} ({self.account.full_code}) {self.amount}"

    def save(self, *args, **kwargs):
        if self.amount.amount == 0:
            raise exceptions.ZeroAmountError()

        leg = super(Leg, self).save(*args, **kwargs)
        mysql_simulate_trigger("check_leg", self.transaction_id)
        return leg

    def natural_key(self):
        return (self.uuid,)

    @property
    def type(self):
        if self.amount.amount < 0:
            return DEBIT
        elif self.amount.amount > 0:
            return CREDIT
        else:
            # This should have been caught earlier by the database integrity check.
            # If you are seeing this then something is wrong with your DB checks.
            raise exceptions.ZeroAmountError()

    def is_debit(self):
        return self.type == DEBIT

    def is_credit(self):
        return self.type == CREDIT

    def account_balance_after(self):
        """Get the balance of the account associated with this leg following the transaction"""
        # TODO: Consider moving to annotation,
        #       particularly once we can count on Django 1.11's subquery support
        #       Or use the new LegView.
        transaction_date = self.transaction.date
        return self.account.balance(
            leg_query=(
                models.Q(transaction__date__lt=transaction_date)
                | (
                    models.Q(transaction__date=transaction_date)
                    & models.Q(transaction_id__lte=self.transaction_id)
                )
            )
        )

    def account_balance_before(self):
        """Get the balance of the account associated with this leg before the transaction"""
        # TODO: Consider moving to annotation,
        #       particularly once we can count on Django 1.11's subquery support.
        #       Or use the new LegView.
        transaction_date = self.transaction.date
        return self.account.balance(
            leg_query=(
                models.Q(transaction__date__lt=transaction_date)
                | (
                    models.Q(transaction__date=transaction_date)
                    & models.Q(transaction_id__lt=self.transaction_id)
                )
            )
        )

    class Meta:
        verbose_name = _("Leg")


class StatementImportManager(models.Manager):
    def get_by_natural_key(self, uuid):
        return self.get(uuid=uuid)


class StatementImport(models.Model):
    """Records an import of a bank statement

    Attributes:

        uuid (UUID): UUID for statement import. Use to prevent leaking of IDs (if desired).
        timestamp (datetime): The datetime when the object was created.
        bank_account (Account): The account the import is for (should normally point to an asset
            account which represents your bank account)

    """

    uuid = models.UUIDField(
        default=UUID_DEFAULT, editable=False, verbose_name=_("uuid")
    )
    timestamp = models.DateTimeField(default=timezone.now, verbose_name=_("timestamp"))
    # TODO: Add constraint to ensure destination account expects statements (copy 0007)
    bank_account = models.ForeignKey(
        Account,
        related_name="imports",
        on_delete=models.CASCADE,
        verbose_name=_("bank account"),
    )
    source = models.CharField(
        max_length=20,
        help_text="A value uniquely identifying where this data came from. "
        'Examples: "csv", "teller.io".',
        verbose_name=_("source"),
    )
    extra = JSONField(
        default=json_default,
        help_text="Any extra data relating to the import, probably specific "
        "to the data source.",
        verbose_name=_("extra"),
    )

    objects = StatementImportManager()

    def natural_key(self):
        return (self.uuid,)

    class Meta:
        verbose_name = _("statementImport")


class StatementLineManager(models.Manager):
    def get_by_natural_key(self, uuid):
        return self.get(uuid=uuid)


class StatementLine(models.Model):
    """Records an single imported bank statement line

    A StatementLine is purely a utility to aid in the creation of transactions
    (in the process known as reconciliation). StatementLines have no impact on
    account balances.

    However, the :meth:`StatementLine.create_transaction()` method can be used to create
    a transaction based on the information in the StatementLine.

    Attributes:

        uuid (UUID): UUID for statement line. Use to prevent leaking of IDs (if desired).
        timestamp (datetime): The datetime when the object was created.
        date (date): The date given by the statement line
        statement_import (StatementImport): The import to which the line belongs
        amount (Decimal): The amount for the statement line, positive or nagative.
        description (str): Any description/memo information provided
        transaction (Transaction): Optionally, the transaction created for this statement line. This normally
            occurs during reconciliation. See also :meth:`StatementLine.create_transaction()`.
    """

    uuid = models.UUIDField(
        default=UUID_DEFAULT, editable=False, verbose_name=_("uuid")
    )
    timestamp = models.DateTimeField(default=timezone.now, verbose_name=_("timestamp"))
    date = models.DateField(verbose_name=_("date"))
    statement_import = models.ForeignKey(
        StatementImport,
        related_name="lines",
        on_delete=models.CASCADE,
        verbose_name=_("statement import"),
    )
    amount = models.DecimalField(
        max_digits=MAX_DIGITS, decimal_places=DECIMAL_PLACES, verbose_name=_("amount")
    )
    description = models.TextField(
        default="", blank=True, verbose_name=_("description")
    )
    type = models.CharField(max_length=50, default="", verbose_name=_("type"))
    # TODO: Add constraint to ensure transaction amount = statement line amount
    # TODO: Add constraint to ensure one statement line per transaction
    transaction = models.ForeignKey(
        Transaction,
        default=None,
        blank=True,
        null=True,
        help_text="Reconcile this statement line to this transaction",
        on_delete=models.SET_NULL,
        verbose_name=_("transaction"),
    )
    source_data = JSONField(
        default=json_default,
        help_text="Original data received from the data source.",
        verbose_name=_("source data"),
    )

    objects = StatementLineManager()

    def natural_key(self):
        return (self.uuid,)

    @property
    def is_reconciled(self):
        """Has this statement line been reconciled?

        Determined as ``True`` if :attr:`transaction` has been set.

        Returns:
            bool: ``True`` if reconciled, ``False`` if not.
        """
        return bool(self.transaction)

    @db_transaction.atomic()
    def create_transaction(self, to_account):
        """Create a transaction for this statement amount and account, into to_account

        This will also set this StatementLine's ``transaction`` attribute to the newly
        created transaction.

        Args:
            to_account (Account): The account the transaction is into / out of.

        Returns:
            Transaction: The newly created (and committed) transaction.

        """
        from_account = self.statement_import.bank_account

        transaction = Transaction.objects.create()
        Leg.objects.create(
            transaction=transaction, account=from_account, amount=+(self.amount * -1)
        )
        Leg.objects.create(
            transaction=transaction, account=to_account, amount=-(self.amount * -1)
        )

        transaction.date = self.date
        transaction.save()

        self.transaction = transaction
        self.save()
        return transaction

    class Meta:
        verbose_name = _("statementLine")


def mysql_simulate_trigger(proc_name, *args):
    # MySQL/MariaDB does not support deferred constraint triggers (unlike postgres),
    # and also does not support triggers updating the table they are triggered from.
    # So this function allows us to trigger manual function calls on transaction finish.
    # Enforcing this at the application level is not idea. If this is important to you
    # then use postgres.
    # (https://stackoverflow.com/a/15300941/1908381)
    def _mysql_call_proc():
        with connection.cursor() as curs:
            curs.callproc(proc_name, args)

    if connection.vendor == "mysql":
        with connection.cursor():
            transaction.on_commit(_mysql_call_proc)