Add conversions between String and Id<T>.

There's already an implicit conversion (via serialization), it's just awkward to use. However, we now need those conversions more directly.

1 files changed, 65 insertions, 24 deletions

diff --git a/src/id.rs b/src/id.rs
index 8d71c5b..86cdf8a 100644
--- a/src/id.rs
+++ b/src/id.rs
@@ -1,8 +1,9 @@
+use std::fmt;
+
 use rand::{seq::SliceRandom, thread_rng};
 use serde::{Deserializer, Serializer};
 use sqlx::encode::IsNull;
 use sqlx::{Database, Decode, Encode, Type};
-use std::fmt;
 
 // Make IDs that:
 //
@@ -58,12 +59,10 @@ const ID_SIZE: usize = 15;
 #[derive(Clone, Debug, Hash, Eq, Ord, PartialEq, PartialOrd)]
 pub struct Id<T>(String, T);
 
-impl<T> fmt::Display for Id<T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.0.fmt(f)
-    }
-}
-
+// Implementations of prefix provide type tokens for IDs where a new value can be generated, with a
+// known prefix. Most implementations should also implement Default and be zero-sized types without
+// state, as many of the operations on Ids will discard or re-create type tokens (using
+// `Default::default()`) when converting between an ID type and some external form.
 pub trait Prefix {
     fn prefix(&self) -> &str;
 }
@@ -86,16 +85,57 @@ where
     }
 }
 
+impl<T> Id<T> {
+    pub fn as_str(&self) -> &str {
+        let Self(value, _) = self;
+        value
+    }
+
+    fn inner(&self) -> &String {
+        let Self(value, _) = self;
+        value
+    }
+}
+
+// Any ID can be converted to a string. Its type token is discarded during the conversion.
+impl<T> From<Id<T>> for String {
+    fn from(value: Id<T>) -> Self {
+        let Id(value, _) = value;
+        value
+    }
+}
+
+// If the type token implements `Default`, then the corresponding ID type can be converted from a
+// string. A new type token will be generated using `T::default()`.
+impl<T> From<String> for Id<T>
+where
+    T: Default,
+{
+    fn from(value: String) -> Self {
+        Self(value, T::default())
+    }
+}
+
+// IDs are printable, exactly as their contained string is printable.
+impl<T> fmt::Display for Id<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.inner().fmt(f)
+    }
+}
+
+// IDs are serializable, exactly as their contained string is serializable. The type token is
+// discarded during serialization.
 impl<T> serde::ser::Serialize for Id<T> {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where
         S: Serializer,
     {
-        let Self(id, _) = self;
-        id.serialize(serializer)
+        self.inner().serialize(serializer)
     }
 }
 
+// If the type token implements `Default`, then the corresponding ID type is deserializable as a
+// String. A new type token will be generated using `T::default()` during deserialization.
 impl<'de, T> serde::de::Deserialize<'de> for Id<T>
 where
     T: Default,
@@ -104,16 +144,18 @@ where
     where
         D: Deserializer<'de>,
     {
-        let instance = T::default();
         let id = String::deserialize(deserializer)?;
 
-        Ok(Self(id, instance))
+        Ok(id.into())
     }
 }
 
-// Type is manually implemented so that we can implement Decode to do
-// recover the type token on read. Implementation is otherwise based on
-// `#[derive(sqlx::Type)]` with the `#[sqlx(transparent)]` attribute.
+// An ID is an sqlx database type for any database where String is a database type. In practice,
+// that's all of them.
+//
+// This is largely cribbed from the implementation generated by `#[sqlx::transparent]`, but we
+// implement it by hand here in order to handle the fact that an ID has two contained values,
+// rather than one.
 impl<DB, T> Type<DB> for Id<T>
 where
     DB: Database,
@@ -128,6 +170,8 @@ where
     }
 }
 
+// If the type token implements Default, then the corresponding ID type can be decoded as a String
+// from a database record. A new type token will be generated using `T::default()` during decoding.
 impl<'r, DB, T> Decode<'r, DB> for Id<T>
 where
     DB: Database,
@@ -135,12 +179,13 @@ where
     T: Default,
 {
     fn decode(value: <DB as Database>::ValueRef<'r>) -> Result<Self, sqlx::error::BoxDynError> {
-        let instance = T::default();
         let id = String::decode(value)?;
-        Ok(Self(id, instance))
+        Ok(id.into())
     }
 }
 
+// An ID is encodeable to a database value whenever String is encodeable. The type token is
+// discarded during encoding.
 impl<'q, DB, T> Encode<'q, DB> for Id<T>
 where
     DB: Database,
@@ -150,25 +195,21 @@ where
         self,
         buf: &mut <DB as Database>::ArgumentBuffer<'q>,
     ) -> Result<IsNull, sqlx::error::BoxDynError> {
-        let Self(id, _) = self;
-        id.encode(buf)
+        self.inner().encode(buf)
     }
 
     fn encode_by_ref(
         &self,
         buf: &mut <DB as Database>::ArgumentBuffer<'q>,
     ) -> Result<IsNull, sqlx::error::BoxDynError> {
-        let Self(id, _) = self;
-        id.encode_by_ref(buf)
+        self.inner().encode_by_ref(buf)
     }
 
     fn produces(&self) -> Option<<DB as Database>::TypeInfo> {
-        let Self(id, _) = self;
-        id.produces()
+        self.inner().produces()
     }
 
     fn size_hint(&self) -> usize {
-        let Self(id, _) = self;
-        id.size_hint()
+        self.inner().size_hint()
     }
 }