fix transactions

patch-db/Cargo.toml

@@ -19,3 +19,6 @@ serde_json = "1.0.61"
 serde_cbor = { git = "https://github.com/dr-bonez/cbor.git" }
 thiserror = "1.0.23"
 tokio = { version = "1.0.1", features = ["sync", "fs", "rt", "io-util", "macros"] }
+
+[dev-dependencies]
+serde = { version = "1.0.118", features = ["rc", "derive"] }

patch-db/src/lib.rs

@@ -7,7 +7,7 @@ use std::sync::Arc;
 
 use fd_lock_rs::FdLock;
 use futures::future::{BoxFuture, FutureExt};
-use json_patch::{Patch, PatchOperation};
+use json_patch::{AddOperation, Patch, PatchOperation, RemoveOperation, ReplaceOperation};
 use json_ptr::{JsonPointer, SegList};
 use qutex_2::{QrwLock, ReadGuard, WriteGuard};
 use serde::{Deserialize, Serialize};
@@ -16,11 +16,13 @@ use thiserror::Error;
 use tokio::{
     fs::File,
     sync::{
-        broadcast::{Receiver, Sender},
-        Mutex, RwLock,
+        broadcast::{error::TryRecvError, Receiver, Sender},
+        Mutex, RwLock, RwLockReadGuard, RwLockWriteGuard,
     },
 };
 
+// note: inserting into an array (before another element) without proper locking can result in unexpected behaviour
+
 #[cfg(test)]
 mod test;
 
@@ -42,18 +44,146 @@ pub enum Error {
     FDLock(#[from] fd_lock_rs::Error),
     #[error("Database Cache Corrupted: {0}")]
     CacheCorrupted(Arc<IOError>),
-    #[error("Mutex Error (should be unreachable): {0}")]
-    MutexError(#[from] tokio::sync::TryLockError),
+    #[error("Subscriber Error: {0}")]
+    Subscriber(#[from] TryRecvError),
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize)]
 pub struct Revision {
     pub id: u64,
-    pub patch: Patch,
+    pub patch: DiffPatch,
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Deserialize, Serialize)]
 pub struct DiffPatch(Patch);
+impl DiffPatch {
+    // safe to assume dictionary style symantics for arrays since patches will always be rebased before being applied
+    pub fn for_path<S: AsRef<str>, V: SegList>(&self, ptr: &JsonPointer<S, V>) -> DiffPatch {
+        let DiffPatch(Patch(ops)) = self;
+        DiffPatch(Patch(
+            ops.iter()
+                .filter_map(|op| match op {
+                    PatchOperation::Add(op) => {
+                        if let Some(tail) = op.path.strip_prefix(ptr) {
+                            Some(PatchOperation::Add(AddOperation {
+                                path: tail.to_owned(),
+                                value: op.value.clone(),
+                            }))
+                        } else if let Some(tail) = ptr.strip_prefix(&op.path) {
+                            Some(PatchOperation::Add(AddOperation {
+                                path: Default::default(),
+                                value: tail.get(&op.value).cloned().unwrap_or_default(),
+                            }))
+                        } else {
+                            None
+                        }
+                    }
+                    PatchOperation::Replace(op) => {
+                        if let Some(tail) = op.path.strip_prefix(ptr) {
+                            Some(PatchOperation::Replace(ReplaceOperation {
+                                path: tail.to_owned(),
+                                value: op.value.clone(),
+                            }))
+                        } else if let Some(tail) = ptr.strip_prefix(&op.path) {
+                            Some(PatchOperation::Replace(ReplaceOperation {
+                                path: Default::default(),
+                                value: tail.get(&op.value).cloned().unwrap_or_default(),
+                            }))
+                        } else {
+                            None
+                        }
+                    }
+                    PatchOperation::Remove(op) => {
+                        if ptr.starts_with(&op.path) {
+                            Some(PatchOperation::Replace(ReplaceOperation {
+                                path: Default::default(),
+                                value: Default::default(),
+                            }))
+                        } else if let Some(tail) = op.path.strip_prefix(ptr) {
+                            Some(PatchOperation::Remove(RemoveOperation {
+                                path: tail.to_owned(),
+                            }))
+                        } else {
+                            None
+                        }
+                    }
+                    _ => unreachable!(),
+                })
+                .collect(),
+        ))
+    }
+    pub fn rebase(&mut self, onto: &DiffPatch) {
+        let DiffPatch(Patch(ops)) = self;
+        let DiffPatch(Patch(onto_ops)) = onto;
+        for onto_op in onto_ops {
+            if let PatchOperation::Add(onto_op) = onto_op {
+                let arr_path_idx = onto_op.path.len() - 1;
+                if let Some(onto_idx) = onto_op
+                    .path
+                    .get_segment(arr_path_idx)
+                    .and_then(|seg| seg.parse::<usize>().ok())
+                {
+                    let prefix = onto_op.path.slice(..arr_path_idx).unwrap_or_default();
+                    for op in ops.iter_mut() {
+                        let path = match op {
+                            PatchOperation::Add(op) => &mut op.path,
+                            PatchOperation::Replace(op) => &mut op.path,
+                            PatchOperation::Remove(op) => &mut op.path,
+                            _ => unreachable!(),
+                        };
+                        if path.starts_with(&prefix) {
+                            if let Some(idx) = path
+                                .get_segment(arr_path_idx)
+                                .and_then(|seg| seg.parse::<usize>().ok())
+                            {
+                                if idx >= onto_idx {
+                                    let mut new_path = prefix.clone().to_owned();
+                                    new_path.push_end_idx(idx + 1);
+                                    if let Some(tail) = path.slice(arr_path_idx + 1..) {
+                                        new_path.append(&tail);
+                                    }
+                                    *path = new_path;
+                                }
+                            }
+                        }
+                    }
+                }
+            } else if let PatchOperation::Remove(onto_op) = onto_op {
+                let arr_path_idx = onto_op.path.len() - 1;
+                if let Some(onto_idx) = onto_op
+                    .path
+                    .get_segment(arr_path_idx)
+                    .and_then(|seg| seg.parse::<usize>().ok())
+                {
+                    let prefix = onto_op.path.slice(..arr_path_idx).unwrap_or_default();
+                    for op in ops.iter_mut() {
+                        let path = match op {
+                            PatchOperation::Add(op) => &mut op.path,
+                            PatchOperation::Replace(op) => &mut op.path,
+                            PatchOperation::Remove(op) => &mut op.path,
+                            _ => unreachable!(),
+                        };
+                        if path.starts_with(&prefix) {
+                            if let Some(idx) = path
+                                .get_segment(arr_path_idx)
+                                .and_then(|seg| seg.parse::<usize>().ok())
+                            {
+                                if idx >= onto_idx {
+                                    let mut new_path = prefix.clone().to_owned();
+                                    new_path.push_end_idx(idx - 1);
+                                    if let Some(tail) = path.slice(arr_path_idx + 1..) {
+                                        new_path.append(&tail);
+                                    }
+                                    *path = new_path;
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
 
 pub struct Store {
     file: FdLock<File>,
@@ -176,7 +306,7 @@ impl Store {
 
         let id = self.revision;
         self.revision += 1;
-        let res = Arc::new(Revision { id, patch: patch.0 });
+        let res = Arc::new(Revision { id, patch });
 
         Ok(res)
     }
@@ -211,8 +341,19 @@ impl PatchDb {
     ) -> Result<Arc<Revision>, Error> {
         self.store.write().await.put(ptr, value).await
     }
-    pub async fn apply(&self, patch: DiffPatch) -> Result<Arc<Revision>, Error> {
-        self.store.write().await.apply(patch).await
+    pub async fn apply(
+        &self,
+        patch: DiffPatch,
+        store_write_lock: Option<RwLockWriteGuard<'_, Store>>,
+    ) -> Result<Arc<Revision>, Error> {
+        let mut store = if let Some(store_write_lock) = store_write_lock {
+            store_write_lock
+        } else {
+            self.store.write().await
+        };
+        let rev = store.apply(patch).await?;
+        self.subscriber.send(rev.clone()).unwrap_or_default(); // ignore errors
+        Ok(rev)
     }
     pub fn subscribe(&self) -> Receiver<Arc<Revision>> {
         self.subscriber.subscribe()
@@ -222,20 +363,24 @@ impl PatchDb {
             db: self.clone(),
             locks: Vec::new(),
             updates: DiffPatch(Patch(Vec::new())),
+            sub: self.subscribe(),
         }
     }
 }
-pub trait Checkpoint {
-    type SubTx: Checkpoint;
+pub trait Checkpoint: Sized {
+    fn rebase(&mut self) -> Result<(), Error>;
     fn get_value<'a, S: AsRef<str> + Send + Sync + 'a, V: SegList + Send + Sync + 'a>(
         &'a mut self,
         ptr: &'a JsonPointer<S, V>,
+        store_read_lock: Option<RwLockReadGuard<'a, Store>>,
     ) -> BoxFuture<'a, Result<Value, Error>>;
     fn put_value<'a, S: AsRef<str> + Send + Sync + 'a, V: SegList + Send + Sync + 'a>(
         &'a mut self,
         ptr: &'a JsonPointer<S, V>,
         value: &'a Value,
     ) -> BoxFuture<'a, Result<(), Error>>;
+    fn store(&self) -> Arc<RwLock<Store>>;
+    fn subscribe(&self) -> Receiver<Arc<Revision>>;
     fn locker_and_locks(&mut self) -> (&Locker, Vec<&mut [(JsonPointer, LockerGuard)]>);
     fn apply(&mut self, patch: DiffPatch);
     fn lock<'a, S: AsRef<str> + Clone + Send + Sync, V: SegList + Clone + Send + Sync>(
@@ -269,36 +414,35 @@ pub struct Transaction {
     db: PatchDb,
     locks: Vec<(JsonPointer, LockerGuard)>,
     updates: DiffPatch,
+    sub: Receiver<Arc<Revision>>,
 }
 impl Transaction {
+    pub fn rebase(&mut self) -> Result<(), Error> {
+        while let Some(rev) = match self.sub.try_recv() {
+            Ok(a) => Some(a),
+            Err(TryRecvError::Empty) => None,
+            Err(e) => return Err(e.into()),
+        } {
+            self.updates.rebase(&rev.patch);
+        }
+        Ok(())
+    }
     async fn get_value<S: AsRef<str>, V: SegList>(
         &mut self,
         ptr: &JsonPointer<S, V>,
+        store_read_lock: Option<RwLockReadGuard<'_, Store>>,
     ) -> Result<Value, Error> {
-        let mut data: Value = ptr
-            .get(self.db.store.read().await.get_data()?)
-            .unwrap_or(&Value::Null)
-            .clone();
-        for op in (self.updates.0).0.iter() {
-            match op {
-                PatchOperation::Add(ref op) => {
-                    if let Some(path) = op.path.strip_prefix(ptr) {
-                        path.insert(&mut data, op.value.clone(), false)?;
-                    }
-                }
-                PatchOperation::Remove(ref op) => {
-                    if let Some(path) = op.path.strip_prefix(ptr) {
-                        path.remove(&mut data, false);
-                    }
-                }
-                PatchOperation::Replace(ref op) => {
-                    if let Some(path) = op.path.strip_prefix(ptr) {
-                        path.set(&mut data, op.value.clone(), false)?;
-                    }
-                }
-                _ => unreachable!("Diff patches cannot contain other operations."),
-            }
-        }
+        let mut data = {
+            let store_lock = self.db.store.clone();
+            let store = if let Some(store_read_lock) = store_read_lock {
+                store_read_lock
+            } else {
+                store_lock.read().await
+            };
+            self.rebase()?;
+            ptr.get(store.get_data()?).cloned().unwrap_or_default()
+        };
+        json_patch::patch(&mut data, &self.updates.for_path(ptr).0)?;
         Ok(data)
     }
     async fn put_value<S: AsRef<str>, V: SegList>(
@@ -306,8 +450,8 @@ impl Transaction {
         ptr: &JsonPointer<S, V>,
         value: &Value,
     ) -> Result<(), Error> {
-        let old = Transaction::get_value(self, ptr).await?;
-        let mut patch = json_patch::diff(&old, value);
+        let old = Transaction::get_value(self, ptr, None).await?;
+        let mut patch = json_patch::diff(&old, &value);
         patch.prepend(ptr);
         (self.updates.0).0.extend(patch.0);
         Ok(())
@@ -340,7 +484,7 @@ impl Transaction {
     ) -> Result<T, Error> {
         self.lock(ptr, lock).await;
         Ok(serde_json::from_value(
-            Transaction::get_value(self, ptr).await?,
+            Transaction::get_value(self, ptr, None).await?,
         )?)
     }
     pub async fn put<T: Serialize, S: AsRef<str>, V: SegList>(
@@ -350,17 +494,36 @@ impl Transaction {
     ) -> Result<(), Error> {
         Transaction::put_value(self, ptr, &serde_json::to_value(value)?).await
     }
-    pub async fn commit(self) -> Result<Arc<Revision>, Error> {
-        self.db.apply(self.updates).await
+    pub async fn commit(mut self) -> Result<Arc<Revision>, Error> {
+        let store_lock = self.db.store.clone();
+        let store = store_lock.write().await;
+        self.rebase()?;
+        self.db.apply(self.updates, Some(store)).await
+    }
+    pub async fn begin(&mut self) -> Result<SubTransaction<&mut Self>, Error> {
+        let store_lock = self.db.store.clone();
+        let store = store_lock.read().await;
+        self.rebase()?;
+        let sub = self.db.subscribe();
+        drop(store);
+        Ok(SubTransaction {
+            parent: self,
+            locks: Vec::new(),
+            updates: DiffPatch(Patch(Vec::new())),
+            sub,
+        })
     }
 }
 impl<'a> Checkpoint for &'a mut Transaction {
-    type SubTx = &'a mut SubTransaction<Self>;
+    fn rebase(&mut self) -> Result<(), Error> {
+        Transaction::rebase(self)
+    }
     fn get_value<'b, S: AsRef<str> + Send + Sync + 'b, V: SegList + Send + Sync + 'b>(
         &'b mut self,
         ptr: &'b JsonPointer<S, V>,
+        store_read_lock: Option<RwLockReadGuard<'b, Store>>,
     ) -> BoxFuture<'b, Result<Value, Error>> {
-        Transaction::get_value(self, ptr).boxed()
+        Transaction::get_value(self, ptr, store_read_lock).boxed()
     }
     fn put_value<'b, S: AsRef<str> + Send + Sync + 'b, V: SegList + Send + Sync + 'b>(
         &'b mut self,
@@ -369,6 +532,12 @@ impl<'a> Checkpoint for &'a mut Transaction {
     ) -> BoxFuture<'b, Result<(), Error>> {
         Transaction::put_value(self, ptr, value).boxed()
     }
+    fn store(&self) -> Arc<RwLock<Store>> {
+        self.db.store.clone()
+    }
+    fn subscribe(&self) -> Receiver<Arc<Revision>> {
+        self.db.subscribe()
+    }
     fn locker_and_locks(&mut self) -> (&Locker, Vec<&mut [(JsonPointer, LockerGuard)]>) {
         (&self.db.locker, vec![&mut self.locks])
     }
@@ -412,33 +581,36 @@ pub struct SubTransaction<Tx: Checkpoint> {
     parent: Tx,
     locks: Vec<(JsonPointer, LockerGuard)>,
     updates: DiffPatch,
+    sub: Receiver<Arc<Revision>>,
 }
-impl<Tx: Checkpoint> SubTransaction<Tx> {
+impl<Tx: Checkpoint + Send + Sync> SubTransaction<Tx> {
+    pub fn rebase(&mut self) -> Result<(), Error> {
+        self.parent.rebase()?;
+        while let Some(rev) = match self.sub.try_recv() {
+            Ok(a) => Some(a),
+            Err(TryRecvError::Empty) => None,
+            Err(e) => return Err(e.into()),
+        } {
+            self.updates.rebase(&rev.patch);
+        }
+        Ok(())
+    }
     async fn get_value<S: AsRef<str> + Send + Sync, V: SegList + Send + Sync>(
         &mut self,
         ptr: &JsonPointer<S, V>,
+        store_read_lock: Option<RwLockReadGuard<'_, Store>>,
     ) -> Result<Value, Error> {
-        let mut data: Value = self.parent.get_value(ptr).await?;
-        for op in (self.updates.0).0.iter() {
-            match op {
-                PatchOperation::Add(ref op) => {
-                    if let Some(path) = op.path.strip_prefix(ptr) {
-                        path.insert(&mut data, op.value.clone(), false)?;
-                    }
-                }
-                PatchOperation::Remove(ref op) => {
-                    if let Some(path) = op.path.strip_prefix(ptr) {
-                        path.remove(&mut data, false);
-                    }
-                }
-                PatchOperation::Replace(ref op) => {
-                    if let Some(path) = op.path.strip_prefix(ptr) {
-                        path.set(&mut data, op.value.clone(), false)?;
-                    }
-                }
-                _ => unreachable!("Diff patches cannot contain other operations."),
-            }
-        }
+        let mut data = {
+            let store_lock = self.parent.store();
+            let store = if let Some(store_read_lock) = store_read_lock {
+                store_read_lock
+            } else {
+                store_lock.read().await
+            };
+            self.rebase()?;
+            self.parent.get_value(ptr, Some(store)).await?
+        };
+        json_patch::patch(&mut data, &self.updates.for_path(ptr).0)?;
         Ok(data)
     }
     async fn put_value<S: AsRef<str> + Send + Sync, V: SegList + Send + Sync>(
@@ -446,8 +618,8 @@ impl<Tx: Checkpoint> SubTransaction<Tx> {
         ptr: &JsonPointer<S, V>,
         value: &Value,
     ) -> Result<(), Error> {
-        let old = SubTransaction::get_value(self, ptr).await?;
-        let mut patch = json_patch::diff(&old, value);
+        let old = SubTransaction::get_value(self, ptr, None).await?;
+        let mut patch = json_patch::diff(&old, &value);
         patch.prepend(ptr);
         (self.updates.0).0.extend(patch.0);
         Ok(())
@@ -482,7 +654,7 @@ impl<Tx: Checkpoint> SubTransaction<Tx> {
     ) -> Result<T, Error> {
         self.lock(ptr, lock).await;
         Ok(serde_json::from_value(
-            SubTransaction::get_value(self, ptr).await?,
+            SubTransaction::get_value(self, ptr, None).await?,
         )?)
     }
     pub async fn put<T: Serialize, S: AsRef<str> + Send + Sync, V: SegList + Send + Sync>(
@@ -490,24 +662,40 @@ impl<Tx: Checkpoint> SubTransaction<Tx> {
         ptr: &JsonPointer<S, V>,
         value: &T,
     ) -> Result<(), Error> {
-        let old = SubTransaction::get_value(self, ptr).await?;
-        let new = serde_json::to_value(value)?;
-        let mut patch = json_patch::diff(&old, &new);
-        patch.prepend(ptr);
-        (self.updates.0).0.extend(patch.0);
+        SubTransaction::put_value(self, ptr, &serde_json::to_value(value)?).await
+    }
+    pub async fn commit(mut self) -> Result<(), Error> {
+        let store_lock = self.parent.store();
+        let store = store_lock.read().await;
+        self.rebase()?;
+        self.parent.apply(self.updates);
+        drop(store);
         Ok(())
     }
-    pub fn commit(mut self) {
-        self.parent.apply(self.updates)
+    pub async fn begin(&mut self) -> Result<SubTransaction<&mut Self>, Error> {
+        let store_lock = self.parent.store();
+        let store = store_lock.read().await;
+        self.rebase()?;
+        let sub = self.parent.subscribe();
+        drop(store);
+        Ok(SubTransaction {
+            parent: self,
+            locks: Vec::new(),
+            updates: DiffPatch(Patch(Vec::new())),
+            sub,
+        })
     }
 }
 impl<'a, Tx: Checkpoint + Send + Sync> Checkpoint for &'a mut SubTransaction<Tx> {
-    type SubTx = &'a mut SubTransaction<Self>;
+    fn rebase(&mut self) -> Result<(), Error> {
+        SubTransaction::rebase(self)
+    }
     fn get_value<'b, S: AsRef<str> + Send + Sync + 'b, V: SegList + Send + Sync + 'b>(
         &'b mut self,
         ptr: &'b JsonPointer<S, V>,
+        store_read_lock: Option<RwLockReadGuard<'b, Store>>,
     ) -> BoxFuture<'b, Result<Value, Error>> {
-        SubTransaction::get_value(self, ptr).boxed()
+        SubTransaction::get_value(self, ptr, store_read_lock).boxed()
     }
     fn put_value<'b, S: AsRef<str> + Send + Sync + 'b, V: SegList + Send + Sync + 'b>(
         &'b mut self,
@@ -516,6 +704,12 @@ impl<'a, Tx: Checkpoint + Send + Sync> Checkpoint for &'a mut SubTransaction<Tx>
     ) -> BoxFuture<'b, Result<(), Error>> {
         SubTransaction::put_value(self, ptr, value).boxed()
     }
+    fn store(&self) -> Arc<RwLock<Store>> {
+        self.parent.store()
+    }
+    fn subscribe(&self) -> Receiver<Arc<Revision>> {
+        self.parent.subscribe()
+    }
     fn locker_and_locks(&mut self) -> (&Locker, Vec<&mut [(JsonPointer, LockerGuard)]>) {
         let (locker, mut locks) = self.parent.locker_and_locks();
         locks.push(&mut self.locks);
@@ -855,15 +1049,22 @@ impl<T> ChildHooks<T> {
     }
 }
 
+pub trait Model {
+    type Data: ModelData;
+}
+
+impl Model for Never {
+    type Data = Never;
+}
+
 pub struct GenericModel<T: Serialize + for<'de> Deserialize<'de>, Parent: ModelData> {
     data: Arc<Mutex<GenericModelData<T, Parent>>>,
     ptr: JsonPointer,
 }
-impl<
-        'a,
-        T: Serialize + for<'de> Deserialize<'de> + Send + Sync + 'static,
-        Parent: ModelData + 'static,
-    > GenericModel<T, Parent>
+impl<T, Parent> GenericModel<T, Parent>
+where
+    T: Serialize + for<'de> Deserialize<'de> + Send + Sync + 'static,
+    Parent: ModelData + 'static,
 {
     pub fn new(ptr: JsonPointer) -> Self {
         Self {
@@ -872,7 +1073,7 @@ impl<
         }
     }
 
-    /// locks
+    // locks
     async fn fetch<Tx: Checkpoint>(&mut self, tx: &mut Tx) -> Result<(), Error> {
         let mut data = self.data.lock().await;
         if let GenericModelData::Uninitialized(children) = &mut *data {
@@ -890,7 +1091,7 @@ impl<
         tx.lock(&self.ptr, lock).await
     }
 
-    /// locks
+    // locks
     pub async fn peek<
         Tx: Checkpoint,
         F: FnOnce(&T) -> ResFut + Send + Sync,
@@ -901,11 +1102,12 @@ impl<
         tx: &mut Tx,
         f: F,
     ) -> Result<Res, Error> {
+        self.lock(tx, LockType::Read).await;
         self.fetch(tx).await?;
         Ok(self.data.lock().await.apply(|t| f(t)).await)
     }
 
-    /// locks
+    // locks
     pub async fn apply<
         Tx: Checkpoint,
         F: FnOnce(&mut T) -> ResFut + Send + Sync,
@@ -916,11 +1118,12 @@ impl<
         tx: &mut Tx,
         f: F,
     ) -> Result<Res, Error> {
+        self.lock(tx, LockType::Write).await;
         self.fetch(tx).await?;
         Ok(self.data.lock().await.apply(f).await)
     }
 
-    /// locks
+    // locks
     pub async fn child<C, F, S, V>(
         &mut self,
         path: &JsonPointer<S, V>,
@@ -1018,3 +1221,11 @@ impl<
         Ok(())
     }
 }
+
+impl<T, Parent> Model for GenericModel<T, Parent>
+where
+    T: Serialize + for<'de> Deserialize<'de> + Send + Sync,
+    Parent: ModelData,
+{
+    type Data = GenericModelData<T, Parent>;
+}

patch-db/src/test.rs

@@ -8,3 +8,25 @@ async fn basic() {
         .await
         .unwrap();
 }
+
+#[derive(Debug, serde::Deserialize, serde::Serialize)]
+pub struct Sample {
+    a: String,
+    b: Child,
+}
+
+pub struct SampleModel<Parent: Model = Never>(GenericModel<Sample, Parent::Data>);
+impl<Parent: Model> Model for SampleModel<Parent> {
+    type Data = GenericModelData<Sample, Parent::Data>;
+}
+
+#[derive(Debug, serde::Deserialize, serde::Serialize)]
+pub struct Child {
+    a: String,
+    b: usize,
+}
+
+pub struct ChildModel<Parent: Model = Never>(GenericModel<Child, Parent::Data>);
+impl<Parent: Model> Model for ChildModel<Parent> {
+    type Data = GenericModelData<Sample, Parent::Data>;
+}