it all works!

src/models.rs

@@ -1,5 +1,7 @@
+use crate::keys::Key;
 use chrono::NaiveDateTime;
 use diesel::prelude::*;
+use eyre::ensure;
 
 #[derive(Queryable, Selectable, Insertable)]
 #[diesel(table_name = crate::schema::accounts)]
@@ -8,3 +10,34 @@ pub struct Account {
     pub email: String,
     pub date: NaiveDateTime,
 }
+
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum OnlyZero {
+    #[default]
+    Zero = 0,
+}
+
+impl From<OnlyZero> for i32 {
+    fn from(_value: OnlyZero) -> Self {
+        0
+    }
+}
+
+impl TryFrom<i32> for OnlyZero {
+    type Error = eyre::Report;
+
+    fn try_from(value: i32) -> Result<Self, Self::Error> {
+        ensure!(value == 0, "tried to convert nonzero value into OnlyZero");
+        Ok(OnlyZero::Zero)
+    }
+}
+
+#[derive(Queryable, Selectable, Insertable, Clone)]
+#[diesel(table_name = crate::schema::keys)]
+#[diesel(check_for_backend(diesel::sqlite::Sqlite))]
+pub struct Keys {
+    #[diesel(serialize_as = i32)]
+    #[diesel(deserialize_as = i32)]
+    pub id: OnlyZero,
+    pub email_unsubscribe_url: Key<ed25519_dalek::SigningKey>,
+}