model: add an index on Post.type

This is useful to quickly browse a list of polymorphic Posts for topics,
programs, etc. The main application is from Member.posts, since
polymorphic collection seems both difficult and edgy.

[MIGRATION] This commit contains a new version of the schema.

app/models/post.py

@@ -10,7 +10,7 @@ class Post(db.Model):
     # Unique Post ID for the whole site
     id = db.Column(db.Integer, primary_key=True)
     # Post type (polymorphic discriminator)
-    type = db.Column(db.String(20))
+    type = db.Column(db.String(20), index=True)
 
     # Creation and edition date
     date_created = db.Column(db.DateTime)

migrations/versions/d2227d2479e2_add_an_index_on_post_type.py

@@ -0,0 +1,28 @@
+"""Add an index on Post.type
+
+Revision ID: d2227d2479e2
+Revises: bcfdb271b88d
+Create Date: 2022-04-25 16:44:51.241965
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'd2227d2479e2'
+down_revision = 'bcfdb271b88d'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_index(op.f('ix_post_type'), 'post', ['type'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_post_type'), table_name='post')
+    # ### end Alembic commands ###