Allow suppliers to have multiple representatives

This adds a basic Representative model and allows for editing them in
the admin interface both in the Suppliers admin panel for each
supplier and in a new Representatives admin panel which allows bulk
editing of all representatives. Currently multiple representatives are
just listed in the component view as extra rows below the company row
but it would probably make sense to add a view for viewing suppliers
directly.

The REST API has also been slightly modified to return a list of
representatives for each supplier in the components endpoint, adding a
suppliers endpoint would probably also be a good idea.

Requires running:
$ python manage.py migrate procurement 0002_add_representative

To update the database for the new model, both forward and lossy reverse
data migration is implemented in the migration.

1 files changed, 80 insertions, 0 deletions

diff --git a/procurement/migrations/0002_add_representative.py b/procurement/migrations/0002_add_representative.py
new file mode 100644
index 0000000..86a2f38
--- /dev/null
+++ b/procurement/migrations/0002_add_representative.py
@@ -0,0 +1,80 @@
+# Generated by Django 2.1.2 on 2018-10-18 02:18
+
+from django.db import migrations, models
+import django.db.models.deletion
+import warnings
+
+def copy_reps_forward(apps, editor):
+    Supplier = apps.get_model("procurement", "Supplier")
+    Representative = apps.get_model("procurement", "Representative")
+    suppliers = Supplier.objects.all()
+    for sup in suppliers:
+        name = sup.representative_name
+        email = sup.representative_email
+        rep = Representative(name=name, email=email, supplier=sup)
+        rep.save()
+
+class ReverseMigrationDataLossWarning(UserWarning):
+    def __init__(self, supplier, kept_rep, lost_reps):
+        self.supplier = supplier
+        self.kept_rep = kept_rep
+        self.lost_reps = lost_reps
+
+    @staticmethod
+    def format_rep(rep):
+        return '{} <{}>'.format(rep.name, rep.email)
+
+    def __str__(self):
+        return "Supplier {} has multiple representatives, only keeping {}. {} will be lost".format(self.supplier.name, self.format_rep(self.kept_rep), [self.format_rep(rep) for rep in self.lost_reps])
+
+def copy_reps_rev(apps, editor):
+    warnings.filterwarnings("always", category=ReverseMigrationDataLossWarning)
+    Supplier = apps.get_model("procurement", "Supplier")
+    Representative = apps.get_model("procurement", "Representative")
+    suppliers = Supplier.objects.all()
+    for sup in suppliers:
+        reps = sup.representatives.all()
+        if reps:
+            keep = reps[0]
+            if len(reps) > 1:
+                lost = reps[1:]
+                warnings.warn(ReverseMigrationDataLossWarning(sup, keep, lost))
+            sup.representative_name = keep.name
+            sup.representative_email = keep.email
+            sup.save()
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('procurement', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Representative',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('created', models.DateTimeField(auto_now_add=True)),
+                ('updated', models.DateTimeField(auto_now=True)),
+                ('name', models.CharField(max_length=255)),
+                ('email', models.CharField(max_length=255)),
+            ],
+            options={
+                'abstract': False,
+            },
+        ),
+        migrations.AddField(
+            model_name='representative',
+            name='supplier',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='representatives', to='procurement.Supplier'),
+        ),
+        migrations.RunPython(copy_reps_forward, copy_reps_rev),
+        migrations.RemoveField(
+            model_name='supplier',
+            name='representative_email',
+        ),
+        migrations.RemoveField(
+            model_name='supplier',
+            name='representative_name',
+        ),
+    ]