[1.7.x] Implement Migration.run_before

This attribute (used for reverse dependencies) was previously declared
and mentioned in the code, but never actually used.

tests/migrations/test_migrations_run_before/0001_initial.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    operations = [
+
+        migrations.CreateModel(
+            "Salamander",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("size", models.IntegerField(default=0)),
+                ("silly_field", models.BooleanField(default=False)),
+            ],
+        ),
+
+    ]

tests/migrations/test_migrations_run_before/0002_second.py

@@ -0,0 +1,23 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("migrations", "0001_initial"),
+    ]
+
+    operations = [
+
+        migrations.CreateModel(
+            "Book",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("author", models.ForeignKey("migrations.Author", null=True)),
+            ],
+        )
+
+    ]

tests/migrations/test_migrations_run_before/0003_third.py

@@ -0,0 +1,32 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    """
+    This is a wee bit crazy, but it's just to show that run_before works.
+    """
+
+    dependencies = [
+        ("migrations", "0001_initial"),
+    ]
+
+    run_before = [
+        ("migrations", "0002_second"),
+    ]
+
+    operations = [
+
+        migrations.CreateModel(
+            "Author",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("name", models.CharField(max_length=255)),
+                ("slug", models.SlugField(null=True)),
+                ("age", models.IntegerField(default=0)),
+            ],
+        )
+
+    ]