billing: Document model fields that are not self explanatory.

corporate/models.py

@@ -22,8 +22,10 @@ class Customer(models.Model):
     default_discount: Optional[Decimal] = models.DecimalField(
         decimal_places=4, max_digits=7, null=True
     )
-    # Some non-profit organizations on manual license management pay only for their paid employees.
-    # We don't prevent these organizations from adding more users than the number of licenses they purchased.
+    # Some non-profit organizations on manual license management pay
+    # only for their paid employees.  We don't prevent these
+    # organizations from adding more users than the number of licenses
+    # they purchased.
     exempt_from_from_license_number_check: bool = models.BooleanField(default=False)
 
     def __str__(self) -> str:
@@ -57,18 +59,39 @@ class CustomerPlan(models.Model):
     # Discount that was applied. For display purposes only.
     discount: Optional[Decimal] = models.DecimalField(decimal_places=4, max_digits=6, null=True)
 
+    # Initialized with the time of plan creation. Used for calculating
+    # start of next billing cycle, next invoice date etc. This value
+    # should never be modified. The only exception is when we change
+    # the status of the plan from free trial to active and reset the
+    # billing_cycle_anchor.
     billing_cycle_anchor: datetime.datetime = models.DateTimeField()
+
     ANNUAL = 1
     MONTHLY = 2
     billing_schedule: int = models.SmallIntegerField()
 
+    # The next date the billing system should go through ledger
+    # entries and create invoices for additional users or plan
+    # renewal. Since we use a daily cron job for invoicing, the
+    # invoice will be generated the first time the cron job runs after
+    # next_invoice_date.
     next_invoice_date: Optional[datetime.datetime] = models.DateTimeField(db_index=True, null=True)
+
+    # On next_invoice_date, we go through ledger entries that were
+    # created after invoiced_through and process them by generating
+    # invoices for any additional users and/or plan renewal. Once the
+    # invoice is generated, we update the value of invoiced_through
+    # and set it to the last ledger entry we processed.
     invoiced_through: Optional["LicenseLedger"] = models.ForeignKey(
         "LicenseLedger", null=True, on_delete=CASCADE, related_name="+"
     )
+
     DONE = 1
     STARTED = 2
     INITIAL_INVOICE_TO_BE_SENT = 3
+    # This status field helps ensure any errors encountered during the
+    # invoicing process do not leave our invoicing system in a broken
+    # state.
     invoicing_status: int = models.SmallIntegerField(default=DONE)
 
     STANDARD = 1
@@ -87,7 +110,8 @@ class CustomerPlan(models.Model):
     NEVER_STARTED = 12
     status: int = models.SmallIntegerField(default=ACTIVE)
 
-    # TODO maybe override setattr to ensure billing_cycle_anchor, etc are immutable
+    # TODO maybe override setattr to ensure billing_cycle_anchor, etc
+    # are immutable.
 
     @property
     def name(self) -> str:
@@ -147,10 +171,21 @@ class LicenseLedger(models.Model):
     """
 
     plan: CustomerPlan = models.ForeignKey(CustomerPlan, on_delete=CASCADE)
+
     # Also True for the initial upgrade.
     is_renewal: bool = models.BooleanField(default=False)
+
     event_time: datetime.datetime = models.DateTimeField()
+
+    # The number of licenses ("seats") purchased by the the organization at the time of ledger
+    # entry creation. Normally, to add a user the organization needs at least one spare license.
+    # Once a license is purchased, it is valid till the end of the billing period, irrespective
+    # of whether the license is used or not. So the value of licenses will never decrease for
+    # subsequent LicenseLedger entries in the same billing period.
     licenses: int = models.IntegerField()
-    # None means the plan does not automatically renew.
-    # This cannot be None if plan.automanage_licenses.
+
+    # The number of licenses the organization needs in the next billing cycle. The value of
+    # licenses_at_next_renewal can increase or decrease for subsequent LicenseLedger entries in
+    # the same billing period. For plans on automatic license management this value is usually
+    # equal to the number of activated users in the organization.
     licenses_at_next_renewal: Optional[int] = models.IntegerField(null=True)