diff --git a/backend/src/controller/judger/mod.rs b/backend/src/controller/judger/mod.rs
index 3707bccd..74f0f63c 100644
--- a/backend/src/controller/judger/mod.rs
+++ b/backend/src/controller/judger/mod.rs
@@ -324,7 +324,7 @@ impl JudgerController {
                 .await
             {
                 Ok(submit) => {
-                    score::ScoreUpload::new(req.user, problem, submit.score, submit.accept)
+                    score::ScoreUpload::new(req.user, problem, submit)
                         .upload(&db)
                         .await;
                 }
diff --git a/backend/src/controller/judger/score.rs b/backend/src/controller/judger/score.rs
index 7e37b8b1..6a7f40d7 100644
--- a/backend/src/controller/judger/score.rs
+++ b/backend/src/controller/judger/score.rs
@@ -1,11 +1,14 @@
-use super::{user_contest, DebugName};
+use std::cmp;
+
+use super::{submit, user_contest, DebugName};
 use crate::{
     entity::{contest, problem, user},
     util::error::Error,
 };
+use chrono::Local;
 use sea_orm::{
-    ActiveModelTrait, ActiveValue, DatabaseConnection, EntityTrait, IntoActiveModel, ModelTrait,
-    TransactionTrait,
+    ActiveModelTrait, ActiveValue, ColumnTrait, DatabaseConnection, EntityTrait, IntoActiveModel,
+    ModelTrait, QueryFilter, QueryOrder, TransactionTrait,
 };
 use tracing::instrument;
 
@@ -14,18 +17,16 @@ const MAX_RETRY: usize = 32;
 pub struct ScoreUpload {
     user_id: i32,
     problem: problem::Model,
-    score: u32,
-    pass: bool,
+    submit: submit::Model,
 }
 
 impl ScoreUpload {
     #[instrument(skip(problem))]
-    pub fn new(user_id: i32, problem: problem::Model, score: u32, pass: bool) -> Self {
+    pub fn new(user_id: i32, problem: problem::Model, submit: submit::Model) -> Self {
         Self {
             user_id,
             problem,
-            score,
-            pass,
+            submit,
         }
     }
     #[instrument(skip(self))]
@@ -58,7 +59,7 @@ impl ScoreUpload {
         let txn = db.begin().await?;
 
         let submit_count = model.submit_count.unwrap().saturating_add(1);
-        let accept_count = match self.pass {
+        let accept_count = match self.submit.accept {
             true => model.accept_count.unwrap().saturating_add(1),
             false => model.accept_count.unwrap(),
         };
@@ -71,15 +72,18 @@ impl ScoreUpload {
         txn.commit().await.map_err(Into::<Error>::into)
     }
     async fn upload_user(&self, db: &DatabaseConnection) -> Result<(), Error> {
-        if !self.pass {
+        if !self.submit.accept {
+            tracing::trace!(reason = "not acceptted", "score_user");
             return Ok(());
         }
         let txn = db.begin().await?;
 
         if self.user_id == self.problem.user_id {
+            tracing::trace!(reason = "problem owner score bypass", "score_user");
             return Ok(());
         }
         if !self.problem.public {
+            tracing::trace!(reason = "private problem score bypass", "score_user");
             return Ok(());
         }
 
@@ -94,7 +98,7 @@ impl ScoreUpload {
         let user_score = user.score;
         let mut user = user.into_active_model();
 
-        user.score = ActiveValue::Set(user_score.saturating_add_unsigned(self.score as u64));
+        user.score = ActiveValue::Set(user_score.saturating_add_unsigned(self.submit.score as u64));
         user.update(&txn).await.map_err(Into::<Error>::into)?;
 
         txn.commit().await.map_err(Into::<Error>::into)
@@ -103,9 +107,16 @@ impl ScoreUpload {
         let txn = db.begin().await?;
 
         if self.user_id == self.problem.user_id {
+            tracing::trace!(reason = "problem owner score bypass", "score_contest");
             return Ok(());
         }
         if self.problem.contest_id.is_none() {
+            tracing::trace!(reason = "not under contest", "score_contest");
+            return Ok(());
+        }
+
+        if self.submit.score == 0 {
+            tracing::trace!(reason = "no score to add", "score_contest");
             return Ok(());
         }
 
@@ -118,7 +129,15 @@ impl ScoreUpload {
             .map_err(Into::<Error>::into)?
             .ok_or(Error::NotInDB(user::Entity::DEBUG_NAME))?;
 
+        let now = Local::now().naive_local();
+
+        if contest.end < now {
+            tracing::trace!(reason = "contest ended", "score_contest");
+            return Ok(());
+        }
+
         if contest.hoster == self.user_id {
+            tracing::trace!(reason = "owner score bypass", "score_contest");
             return Ok(());
         }
 
@@ -126,7 +145,20 @@ impl ScoreUpload {
             .ok_or(Error::Unreachable("user_contest should exist"))?
             .into_active_model();
 
-        linker.score = ActiveValue::Set(self.score);
+        let mut score = linker.score.unwrap();
+
+        let submit = self
+            .problem
+            .find_related(submit::Entity)
+            .filter(submit::Column::UserId.eq(self.user_id))
+            .order_by_desc(submit::Column::Score)
+            .one(&txn)
+            .await?;
+
+        score = score.saturating_sub(submit.map(|x| x.score).unwrap_or_default());
+        score = score.saturating_add(self.submit.score);
+
+        linker.score = ActiveValue::Set(score);
         linker.update(&txn).await.map_err(Into::<Error>::into)?;
 
         txn.commit().await.map_err(Into::<Error>::into)
diff --git a/backend/src/endpoint/contest.rs b/backend/src/endpoint/contest.rs
index 5492af53..4e880026 100644
--- a/backend/src/endpoint/contest.rs
+++ b/backend/src/endpoint/contest.rs
@@ -316,24 +316,4 @@ impl ContestSet for Arc<Server> {
 
         Ok(Response::new(()))
     }
-
-    #[doc = " return up to 10 user"]
-    #[instrument(skip_all, level = "debug")]
-    async fn rank(&self, req: Request<ContestId>) -> Result<Response<Users>, Status> {
-        let (auth, req) = self.parse_request(req).await?;
-
-        let contest: IdModel = Entity::related_read_by_id(&auth, req.id, &self.db).await?;
-
-        let list = user_contest::Entity::find()
-            .filter(user_contest::Column::ContestId.eq(contest.id))
-            .order_by_desc(user_contest::Column::Score)
-            .limit(10)
-            .all(self.db.deref())
-            .await
-            .map_err(Into::<Error>::into)?;
-
-        let list: Vec<UserRank> = list.into_iter().map(|x| x.into()).collect();
-
-        Ok(Response::new(Users { list }))
-    }
 }
diff --git a/backend/src/endpoint/problem.rs b/backend/src/endpoint/problem.rs
index 22b1eed4..a7f39576 100644
--- a/backend/src/endpoint/problem.rs
+++ b/backend/src/endpoint/problem.rs
@@ -24,6 +24,7 @@ impl From<PartialModel> for ProblemInfo {
             title: value.title,
             submit_count: value.submit_count,
             ac_rate: value.ac_rate,
+            difficulty: value.difficulty,
         }
     }
 }
@@ -42,6 +43,7 @@ impl From<Model> for ProblemFullInfo {
                 title: value.title,
                 submit_count: value.submit_count,
                 ac_rate: value.ac_rate,
+                difficulty: value.difficulty,
             },
             author: value.user_id.into(),
         }
diff --git a/backend/src/endpoint/user.rs b/backend/src/endpoint/user.rs
index e9f1790b..f7052045 100644
--- a/backend/src/endpoint/user.rs
+++ b/backend/src/endpoint/user.rs
@@ -91,6 +91,39 @@ impl UserSet for Arc<Server> {
 
         Ok(Response::new(ListUserResponse { list, next_session }))
     }
+    async fn list_by_contest(
+        &self,
+        req: Request<ListByRequest>,
+    ) -> Result<Response<ListUserResponse>, Status> {
+        let (auth, req) = self.parse_request(req).await?;
+
+        let (rev, size) = split_rev(req.size);
+        let size = bound!(size, 64);
+        let offset = bound!(req.offset(), 1024);
+
+        let (pager, models) = match req.request.ok_or(Error::NotInPayload("request"))? {
+            list_by_request::Request::Create(create) => {
+                ParentPaginator::new_fetch(
+                    (0, create.parent_id),
+                    &auth,
+                    size,
+                    offset,
+                    create.start_from_end,
+                    &self.db,
+                )
+                .await
+            }
+            list_by_request::Request::Pager(old) => {
+                let pager: ParentPaginator = self.crypto.decode(old.session)?;
+                pager.fetch(&auth, size, offset, rev, &self.db).await
+            }
+        }?;
+
+        let next_session = self.crypto.encode(pager)?;
+        let list = models.into_iter().map(|x| x.into()).collect();
+
+        Ok(Response::new(ListUserResponse { list, next_session }))
+    }
     #[instrument(skip_all, level = "debug")]
     async fn full_info(&self, _req: Request<UserId>) -> Result<Response<UserFullInfo>, Status> {
         Err(Status::cancelled("deprecated"))
diff --git a/backend/src/entity/announcement.rs b/backend/src/entity/announcement.rs
index e6fb3899..86d538fc 100644
--- a/backend/src/entity/announcement.rs
+++ b/backend/src/entity/announcement.rs
@@ -108,14 +108,14 @@ impl PagerReflect<Entity> for PartialModel {
 
 pub struct PagerTrait;
 
+impl PagerData for PagerTrait {
+    type Data = ();
+}
+
 #[async_trait]
 impl PagerSource for PagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = ();
-
     const TYPE_NUMBER: u8 = 4;
 
     async fn filter(
@@ -131,14 +131,14 @@ pub type Paginator = PkPager<PagerTrait, PartialModel>;
 
 pub struct TextPagerTrait;
 
+impl PagerData for TextPagerTrait {
+    type Data = String;
+}
+
 #[async_trait]
 impl PagerSource for TextPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = String;
-
     const TYPE_NUMBER: u8 = 4;
 
     async fn filter(
@@ -154,14 +154,14 @@ pub type TextPaginator = PkPager<TextPagerTrait, PartialModel>;
 
 pub struct ParentPagerTrait;
 
+impl PagerData for ParentPagerTrait {
+    type Data = (i32, chrono::NaiveDateTime);
+}
+
 #[async_trait]
 impl PagerSource for ParentPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = (i32, chrono::NaiveDateTime);
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
@@ -193,14 +193,14 @@ pub type ParentPaginator = ColPager<ParentPagerTrait, PartialModel>;
 
 pub struct ColPagerTrait;
 
+impl PagerData for ColPagerTrait {
+    type Data = (AnnouncementSortBy, String);
+}
+
 #[async_trait]
 impl PagerSource for ColPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = (AnnouncementSortBy, String);
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
diff --git a/backend/src/entity/chat.rs b/backend/src/entity/chat.rs
index d57919a6..ee7941d3 100644
--- a/backend/src/entity/chat.rs
+++ b/backend/src/entity/chat.rs
@@ -77,14 +77,14 @@ impl PagerReflect<Entity> for Model {
 
 pub struct ParentPagerTrait;
 
+impl PagerData for ParentPagerTrait {
+    type Data = (i32, chrono::NaiveDateTime);
+}
+
 #[async_trait]
 impl PagerSource for ParentPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = (i32, chrono::NaiveDateTime);
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
diff --git a/backend/src/entity/contest.rs b/backend/src/entity/contest.rs
index 74273bd0..1e8890f0 100644
--- a/backend/src/entity/contest.rs
+++ b/backend/src/entity/contest.rs
@@ -1,5 +1,6 @@
 use std::ops::Deref;
 
+use chrono::Local;
 use sea_orm::{DatabaseBackend, Statement};
 
 use crate::{grpc::backend::ContestSortBy, partial_union};
@@ -129,22 +130,22 @@ impl super::ParentalTrait<IdModel> for Entity {
     ) -> Result<IdModel, Error> {
         match user::Model::new_with_auth(auth) {
             Some(user) => {
-                // user.find_related(Entity).select_only().columns(col);
                 let (query, param) = {
                     let builder = db.get_database_backend().get_query_builder();
+                    let now = Local::now().naive_local();
 
                     partial_union!(
-                        [Column::Id, Column::Hoster, Column::Public],
+                        [Column::Id, Column::Hoster, Column::Public, Column::Begin],
                         user.find_related(Entity),
-                        Entity::find().filter(Column::Public.eq(true)),
+                        Entity::find()
+                            .filter(Column::Public.eq(true))
+                            .filter(Column::Begin.lte(now)),
                         Entity::find().filter(Column::Hoster.eq(user.id))
                     )
                     .and_where(Column::Id.eq(id))
                     .build_any(builder.deref())
                 };
 
-                // user.find_related(Entity).into_query()
-
                 IdModel::find_by_statement(Statement::from_sql_and_values(
                     DatabaseBackend::Sqlite,
                     query,
@@ -203,14 +204,14 @@ impl PagerReflect<Entity> for PartialModel {
 
 pub struct TextPagerTrait;
 
+impl PagerData for TextPagerTrait {
+    type Data = String;
+}
+
 #[async_trait]
 impl PagerSource for TextPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = String;
-
     const TYPE_NUMBER: u8 = 4;
 
     async fn filter(
@@ -226,15 +227,14 @@ pub type TextPaginator = PkPager<TextPagerTrait, PartialModel>;
 
 pub struct ColPagerTrait;
 
+impl PagerData for ColPagerTrait {
+    type Data = (ContestSortBy, String);
+}
+
 #[async_trait]
 impl PagerSource for ColPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    // FIXME: we need optional support
-    type Data = (ContestSortBy, String);
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
diff --git a/backend/src/entity/education.rs b/backend/src/entity/education.rs
index 0af3e33d..6cec6590 100644
--- a/backend/src/entity/education.rs
+++ b/backend/src/entity/education.rs
@@ -100,14 +100,14 @@ impl PagerReflect<Entity> for PartialModel {
 
 pub struct PagerTrait;
 
+impl PagerData for PagerTrait {
+    type Data = ();
+}
+
 #[async_trait]
 impl PagerSource for PagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = ();
-
     const TYPE_NUMBER: u8 = 4;
 
     async fn filter(
@@ -123,14 +123,14 @@ pub type Paginator = PkPager<PagerTrait, PartialModel>;
 
 pub struct TextPagerTrait;
 
+impl PagerData for TextPagerTrait {
+    type Data = String;
+}
+
 #[async_trait]
 impl PagerSource for TextPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = String;
-
     const TYPE_NUMBER: u8 = 4;
 
     async fn filter(
@@ -146,14 +146,14 @@ pub type TextPaginator = PkPager<TextPagerTrait, PartialModel>;
 
 pub struct ParentPagerTrait;
 
+impl PagerData for ParentPagerTrait {
+    type Data = i32;
+}
+
 #[async_trait]
 impl PagerSource for ParentPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = i32;
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
diff --git a/backend/src/entity/mod.rs b/backend/src/entity/mod.rs
index 5b486777..6e714a86 100644
--- a/backend/src/entity/mod.rs
+++ b/backend/src/entity/mod.rs
@@ -21,7 +21,7 @@ pub mod util;
 use crate::util::{auth::Auth, error::Error};
 use tonic::async_trait;
 
-use util::paginator::{PagerReflect, PagerSource, PkPager};
+use util::paginator::{PagerData, PagerReflect, PagerSource, PkPager};
 
 pub trait DebugName {
     const DEBUG_NAME: &'static str = "TEMPLATE_DEBUG_NAME";
diff --git a/backend/src/entity/problem.rs b/backend/src/entity/problem.rs
index 2abcb386..3f73f26d 100644
--- a/backend/src/entity/problem.rs
+++ b/backend/src/entity/problem.rs
@@ -242,14 +242,14 @@ pub struct PagerTrait;
 
 pub struct TextPagerTrait;
 
+impl PagerData for TextPagerTrait {
+    type Data = String;
+}
+
 #[async_trait]
 impl PagerSource for TextPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = String;
-
     const TYPE_NUMBER: u8 = 4;
 
     async fn filter(
@@ -266,14 +266,14 @@ pub type TextPaginator = PkPager<TextPagerTrait, PartialModel>;
 
 pub struct ParentPagerTrait;
 
+impl PagerData for ParentPagerTrait {
+    type Data = (i32, f32);
+}
+
 #[async_trait]
 impl PagerSource for ParentPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = (i32, f32);
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
@@ -305,14 +305,14 @@ pub type ParentPaginator = ColPager<ParentPagerTrait, PartialModel>;
 
 pub struct ColPagerTrait;
 
+impl PagerData for ColPagerTrait {
+    type Data = (ProblemSortBy, String);
+}
+
 #[async_trait]
 impl PagerSource for ColPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = (ProblemSortBy, String);
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
diff --git a/backend/src/entity/submit.rs b/backend/src/entity/submit.rs
index 9305328d..217fb58a 100644
--- a/backend/src/entity/submit.rs
+++ b/backend/src/entity/submit.rs
@@ -118,14 +118,14 @@ impl PagerReflect<Entity> for PartialModel {
 
 pub struct ParentPagerTrait;
 
+impl PagerData for ParentPagerTrait {
+    type Data = (i32, chrono::NaiveDateTime);
+}
+
 #[async_trait]
 impl PagerSource for ParentPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = (i32, chrono::NaiveDateTime);
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
@@ -156,14 +156,14 @@ pub type ParentPaginator = ColPager<ParentPagerTrait, PartialModel>;
 
 pub struct ColPagerTrait;
 
+impl PagerData for ColPagerTrait {
+    type Data = chrono::NaiveDateTime;
+}
+
 #[async_trait]
 impl PagerSource for ColPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = chrono::NaiveDateTime;
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
diff --git a/backend/src/entity/test.rs b/backend/src/entity/test.rs
index 9fad56cf..b8b8ded8 100644
--- a/backend/src/entity/test.rs
+++ b/backend/src/entity/test.rs
@@ -92,14 +92,14 @@ impl PagerReflect<Entity> for Model {
 
 pub struct ParentPagerTrait;
 
+impl PagerData for ParentPagerTrait {
+    type Data = (i32, u32);
+}
+
 #[async_trait]
 impl PagerSource for ParentPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = (i32, u32);
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
@@ -130,14 +130,14 @@ pub type ParentPaginator = ColPager<ParentPagerTrait, Model>;
 
 pub struct ColPagerTrait;
 
+impl PagerData for ColPagerTrait {
+    type Data = u32;
+}
+
 #[async_trait]
 impl PagerSource for ColPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = u32;
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
diff --git a/backend/src/entity/user.rs b/backend/src/entity/user.rs
index 0abd61a1..9a492468 100644
--- a/backend/src/entity/user.rs
+++ b/backend/src/entity/user.rs
@@ -1,3 +1,5 @@
+use sea_orm::{QueryOrder, QuerySelect};
+
 use crate::grpc::backend::UserSortBy;
 
 use super::*;
@@ -176,14 +178,14 @@ impl PagerReflect<Entity> for Model {
 
 pub struct TextPagerTrait;
 
+impl PagerData for TextPagerTrait {
+    type Data = String;
+}
+
 #[async_trait]
 impl PagerSource for TextPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = String;
-
     const TYPE_NUMBER: u8 = 4;
 
     async fn filter(
@@ -199,14 +201,14 @@ pub type TextPaginator = PkPager<TextPagerTrait, Model>;
 
 pub struct ColPagerTrait;
 
+impl PagerData for ColPagerTrait {
+    type Data = (UserSortBy, String);
+}
+
 #[async_trait]
 impl PagerSource for ColPagerTrait {
     const ID: <Self::Entity as EntityTrait>::Column = Column::Id;
-
     type Entity = Entity;
-
-    type Data = (UserSortBy, String);
-
     const TYPE_NUMBER: u8 = 8;
 
     async fn filter(
@@ -238,3 +240,107 @@ impl PagerSortSource<Model> for ColPagerTrait {
 }
 
 pub type ColPaginator = ColPager<ColPagerTrait, Model>;
+
+/// ParentPaginator (offset base)
+#[derive(serde::Serialize, serde::Deserialize)]
+pub struct ParentPaginator {
+    pub offset: u64,
+    pub ppk: i32,
+    pub start_from_end: bool,
+}
+
+pub struct ParentSource;
+
+impl PagerData for ParentSource {
+    type Data = (u64, i32);
+}
+
+fn to_order(raw: bool) -> sea_query::Order {
+    match raw {
+        true => sea_query::Order::Desc,
+        false => sea_query::Order::Asc,
+    }
+}
+
+#[async_trait]
+impl util::paginator::Pager for ParentPaginator {
+    type Source = ParentSource;
+    type Reflect = Model;
+
+    async fn fetch(
+        mut self,
+        auth: &Auth,
+        size: u64,
+        offset: u64,
+        rel_dir: bool,
+        db: &DatabaseConnection,
+    ) -> Result<(Self, Vec<Self::Reflect>), Error> {
+        // check user is in contest(or admin)
+        contest::Entity::read_by_id(self.ppk, auth)?
+            .one(db)
+            .await?
+            .ok_or(Error::NotInDB(contest::Entity::DEBUG_NAME))?;
+
+        let result = user_contest::Entity::find()
+            .filter(user_contest::Column::ContestId.eq(self.ppk))
+            .order_by(
+                user_contest::Column::Score,
+                to_order(self.start_from_end ^ rel_dir),
+            )
+            .offset(self.offset + offset)
+            .limit(size)
+            .all(db)
+            .await?;
+
+        let result: Vec<_> = result
+            .load_one(Entity, db)
+            .await?
+            .into_iter()
+            .flatten()
+            .collect();
+
+        self.offset += result.len() as u64;
+        Ok((self, result))
+    }
+    async fn new_fetch(
+        data: <Self::Source as PagerData>::Data,
+        auth: &Auth,
+        size: u64,
+        offset: u64,
+        abs_dir: bool,
+        db: &DatabaseConnection,
+    ) -> Result<(Self, Vec<Self::Reflect>), Error> {
+        let ppk = data.1;
+        // check user is in contest(or admin)
+        contest::Entity::read_by_id(ppk, auth)?
+            .one(db)
+            .await?
+            .ok_or(Error::NotInDB(contest::Entity::DEBUG_NAME))?;
+
+        let result = user_contest::Entity::find()
+            .filter(user_contest::Column::ContestId.eq(data.0))
+            .order_by(user_contest::Column::Score, to_order(abs_dir))
+            .offset(offset)
+            .limit(size)
+            .all(db)
+            .await?;
+
+        let result: Vec<_> = result
+            .load_one(Entity, db)
+            .await?
+            .into_iter()
+            .flatten()
+            .collect();
+
+        let offset = offset + result.len() as u64;
+
+        Ok((
+            ParentPaginator {
+                ppk,
+                offset,
+                start_from_end: abs_dir,
+            },
+            result,
+        ))
+    }
+}
diff --git a/backend/src/entity/util/paginator.rs b/backend/src/entity/util/paginator.rs
index c850b538..efd9980a 100644
--- a/backend/src/entity/util/paginator.rs
+++ b/backend/src/entity/util/paginator.rs
@@ -21,6 +21,34 @@ use sea_orm::{ColumnTrait, EntityTrait, QuerySelect, Select};
 
 use crate::util::error::Error;
 
+#[async_trait]
+pub trait Pager
+where
+    Self: Sized + Serialize + DeserializeOwned,
+{
+    type Source: PagerData;
+    type Reflect: Send;
+    async fn fetch(
+        self,
+        auth: &Auth,
+        size: u64,
+        offset: u64,
+        rel_dir: bool,
+        db: &DatabaseConnection,
+    ) -> Result<(Self, Vec<Self::Reflect>), Error>;
+    async fn new_fetch(
+        data: <Self::Source as PagerData>::Data,
+        auth: &Auth,
+        size: u64,
+        offset: u64,
+        abs_dir: bool,
+        db: &DatabaseConnection,
+    ) -> Result<(Self, Vec<Self::Reflect>), Error>;
+}
+
+pub trait PagerData {
+    type Data: Send + Sized + Serialize + DeserializeOwned;
+}
 /// indicate foreign object is ready for page source
 ///
 /// In `Education` example, we expect ::entity::education::Entity
@@ -28,11 +56,10 @@ use crate::util::error::Error;
 #[async_trait]
 pub trait PagerSource
 where
-    Self: Send,
+    Self: Send + PagerData,
 {
     const ID: <Self::Entity as EntityTrait>::Column;
     type Entity: EntityTrait + DebugName;
-    type Data: Send + Sized + Serialize + DeserializeOwned;
     const TYPE_NUMBER: u8;
     /// filter reconstruction
     async fn filter(
@@ -56,31 +83,6 @@ where
     async fn all(query: Select<E>, db: &DatabaseConnection) -> Result<Vec<Self>, Error>;
 }
 
-#[async_trait]
-pub trait Pager
-where
-    Self: Sized + Serialize + DeserializeOwned,
-{
-    type Source: PagerSource;
-    type Reflect: PagerReflect<<Self::Source as PagerSource>::Entity> + Send;
-    async fn fetch(
-        self,
-        auth: &Auth,
-        size: u64,
-        offset: u64,
-        rel_dir: bool,
-        db: &DatabaseConnection,
-    ) -> Result<(Self, Vec<Self::Reflect>), Error>;
-    async fn new_fetch(
-        data: <Self::Source as PagerSource>::Data,
-        auth: &Auth,
-        size: u64,
-        offset: u64,
-        abs_dir: bool,
-        db: &DatabaseConnection,
-    ) -> Result<(Self, Vec<Self::Reflect>), Error>;
-}
-
 /// compact primary key pager
 #[derive(Serialize, Deserialize)]
 pub struct PkPager<S: PagerSource, R: PagerReflect<S::Entity>> {
@@ -91,13 +93,9 @@ pub struct PkPager<S: PagerSource, R: PagerReflect<S::Entity>> {
     /// original direction
     direction: bool,
     /// data
-    #[serde(bound(
-        deserialize = "<<PkPager<S, R> as Pager>::Source as PagerSource>::Data: DeserializeOwned"
-    ))]
-    #[serde(bound(
-        serialize = "<<PkPager<S, R> as Pager>::Source as PagerSource>::Data: Serialize"
-    ))]
-    data: <<PkPager<S, R> as Pager>::Source as PagerSource>::Data,
+    #[serde(bound(deserialize = "S::Data: DeserializeOwned"))]
+    #[serde(bound(serialize = "S::Data: Serialize"))]
+    data: S::Data,
     #[serde(bound(serialize = ""))]
     #[serde(bound(deserialize = ""))]
     source: PhantomData<S>,
@@ -118,7 +116,7 @@ impl<S: PagerSource, R: PagerReflect<S::Entity>> Pager for PkPager<S, R> {
         offset: u64,
         rel_dir: bool,
         db: &DatabaseConnection,
-    ) -> Result<(Self, Vec<Self::Reflect>), Error> {
+    ) -> Result<(Self, Vec<R>), Error> {
         let paginator = PaginatePkBuilder::default()
             .pk(<S as PagerSource>::ID)
             .include(self.last_direction ^ rel_dir)
@@ -150,7 +148,7 @@ impl<S: PagerSource, R: PagerReflect<S::Entity>> Pager for PkPager<S, R> {
         _offset: u64,
         abs_dir: bool,
         db: &DatabaseConnection,
-    ) -> Result<(Self, Vec<Self::Reflect>), Error> {
+    ) -> Result<(Self, Vec<R>), Error> {
         let query = order_by_bool(
             S::filter(auth, &data, db).await?,
             <S as PagerSource>::ID,
@@ -222,7 +220,7 @@ impl<S: PagerSortSource<R>, R: PagerReflect<S::Entity>> Pager for ColPager<S, R>
         offset: u64,
         rel_dir: bool,
         db: &DatabaseConnection,
-    ) -> Result<(Self, Vec<Self::Reflect>), Error> {
+    ) -> Result<(Self, Vec<R>), Error> {
         let col = S::sort_col(&self.data);
         let val = S::get_val(&self.data);
 
@@ -264,7 +262,7 @@ impl<S: PagerSortSource<R>, R: PagerReflect<S::Entity>> Pager for ColPager<S, R>
         offset: u64,
         abs_dir: bool,
         db: &DatabaseConnection,
-    ) -> Result<(Self, Vec<Self::Reflect>), Error> {
+    ) -> Result<(Self, Vec<R>), Error> {
         let col = S::sort_col(&data);
 
         let query = order_by_bool(
diff --git a/proto/backend.proto b/proto/backend.proto
index dfdf955c..caee0dbd 100644
--- a/proto/backend.proto
+++ b/proto/backend.proto
@@ -342,6 +342,7 @@ message ProblemInfo {
   required ProblemId id = 1;
   required string title = 2;
   required uint32 submit_count = 3;
+  required uint32 difficulty = 5;
   required float ac_rate = 4;
 }
 
@@ -614,9 +615,6 @@ service ContestSet {
 
   rpc Join(JoinContestRequest) returns (google.protobuf.Empty);
   rpc Exit(ContestId) returns (google.protobuf.Empty);
-
-  // return up to 10 user
-  rpc Rank(ContestId) returns (Users);
 }
 
 // User
@@ -694,6 +692,7 @@ message ListUserRequest {
 service UserSet {
   rpc List(ListUserRequest) returns (ListUserResponse);
   rpc SearchByText(TextSearchRequest) returns (ListUserResponse);
+  rpc ListByContest(ListByRequest) returns (ListUserResponse);
   rpc FullInfo(UserId) returns (UserFullInfo);
 
   rpc Create(CreateUserRequest) returns (UserId);