refactor(people): Move relationship answers to answer set

Answers to the relationship questions have been moved from the relationship
to another model RelationshipAnswerSet.  A new answer set is created each
time a user answers the relationship questions.

Resolves #16
Resolves #17
Resolves #18

people/models/relationship.py

@@ -0,0 +1,155 @@
+"""
+Models describing relationships between people.
+"""
+
+import typing
+
+from django.db import models
+from django.urls import reverse
+
+from .person import Person
+
+__all__ = [
+    'RelationshipQuestion',
+    'RelationshipQuestionChoice',
+    'RelationshipAnswerSet',
+    'Relationship',
+]
+
+
+class RelationshipQuestion(models.Model):
+    """
+    Question which may be asked about a relationship.
+    """
+    class Meta:
+        ordering = [
+            'order',
+            'text',
+        ]
+
+    #: Version number of this question - to allow modification without invalidating existing data
+    version = models.PositiveSmallIntegerField(default=1,
+                                               blank=False, null=False)
+
+    #: Text of question
+    text = models.CharField(max_length=255,
+                            blank=False, null=False)
+
+    #: Position of this question in the list
+    order = models.SmallIntegerField(default=0,
+                                     blank=False, null=False)
+
+    @property
+    def choices(self) -> typing.List[typing.List[str]]:
+        """
+        Convert the :class:`RelationshipQuestionChoices` for this question into Django choices.
+        """
+        return [
+            [choice.pk, str(choice)] for choice in self.answers.all()
+        ]
+
+    def __str__(self) -> str:
+        return self.text
+
+
+class RelationshipQuestionChoice(models.Model):
+    """
+    Allowed answer to a :class:`RelationshipQuestion`.
+    """
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(fields=['question', 'text'],
+                                    name='unique_question_answer')
+        ]
+        ordering = [
+            'question__order',
+            'order',
+            'text',
+        ]
+
+    #: Question to which this answer belongs
+    question = models.ForeignKey(RelationshipQuestion, related_name='answers',
+                                 on_delete=models.CASCADE,
+                                 blank=False, null=False)
+
+    #: Text of answer
+    text = models.CharField(max_length=255,
+                            blank=False, null=False)
+
+    #: Position of this answer in the list
+    order = models.SmallIntegerField(default=0,
+                                     blank=False, null=False)
+
+    def __str__(self) -> str:
+        return self.text
+
+
+class Relationship(models.Model):
+    """
+    A directional relationship between two people allowing linked questions.
+    """
+
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(fields=['source', 'target'],
+                                    name='unique_relationship'),
+        ]
+
+    #: Person reporting the relationship
+    source = models.ForeignKey(Person, related_name='relationships_as_source',
+                               on_delete=models.CASCADE,
+                               blank=False, null=False)
+
+    #: Person with whom the relationship is reported
+    target = models.ForeignKey(Person, related_name='relationships_as_target',
+                               on_delete=models.CASCADE,
+                               blank=False, null=False)
+    
+    #: When was this relationship defined?
+    created = models.DateTimeField(auto_now_add=True)
+    
+    #: When was this marked as expired?  Default None means it has not expired
+    expired = models.DateTimeField(blank=True, null=True)
+
+    @property
+    def current_answers(self) -> 'RelationshipAnswerSet':
+        return self.answer_sets.last()
+
+    def get_absolute_url(self):
+        return reverse('people:relationship.detail', kwargs={'pk': self.pk})
+
+    def __str__(self) -> str:
+        return f'{self.source} -> {self.target}'
+
+    @property
+    def reverse(self):
+        """
+        Get the reverse of this relationship.
+
+        @raise Relationship.DoesNotExist: When the reverse relationship is not known
+        """
+        return type(self).objects.get(source=self.target,
+                                      target=self.source)
+
+
+class RelationshipAnswerSet(models.Model):
+    """
+    The answers to the relationship questions at a particular point in time.
+    """
+
+    class Meta:
+        ordering = [
+            'timestamp',
+        ]
+
+    #: Relationship to which this answer set belongs
+    relationship = models.ForeignKey(Relationship,
+                                     on_delete=models.CASCADE,
+                                     related_name='answer_sets',
+                                     blank=False, null=False)
+
+    #: Answers to :class:`RelationshipQuestion`s
+    question_answers = models.ManyToManyField(RelationshipQuestionChoice)
+
+    #: When were these answers collected?
+    timestamp = models.DateTimeField(auto_now_add=True)