fix(NIP-01): allow limits on a per-filter basis

The original implementation of subscription limit applied to the
entire query, instead of the specific filter.  Now, each filter gets
its own query limit.  When a limit is applied, the most recent N
events will be returned, otherwise the default is to return the
earliest events (in order), for all matching events.

src/db.rs

@@ -7,6 +7,7 @@ use crate::hexrange::hex_range;
 use crate::hexrange::HexSearch;
 use crate::nip05;
 use crate::schema::{upgrade_db, STARTUP_SQL};
+use crate::subscription::ReqFilter;
 use crate::subscription::Subscription;
 use crate::utils::is_hex;
 use governor::clock::Clock;
@@ -377,22 +378,18 @@ fn repeat_vars(count: usize) -> String {
     s
 }
 
-/// Create a dynamic SQL query string and params from a subscription.
-fn query_from_sub(sub: &Subscription) -> (String, Vec<Box<dyn ToSql>>) {
+/// Create a dynamic SQL subquery and params from a subscription filter.
+fn query_from_filter(f: &ReqFilter) -> (String, Vec<Box<dyn ToSql>>) {
     // build a dynamic SQL query.  all user-input is either an integer
     // (sqli-safe), or a string that is filtered to only contain
     // hexadecimal characters.  Strings that require escaping (tag
     // names/values) use parameters.
-    let mut limit: Option<u32> = None;
     let mut query =
-        "SELECT DISTINCT(e.content) FROM event e LEFT JOIN tag t ON e.id=t.event_id ".to_owned();
-    // parameters
+        "SELECT DISTINCT(e.content), e.created_at FROM event e LEFT JOIN tag t ON e.id=t.event_id "
+            .to_owned();
+    // query parameters for SQLite
     let mut params: Vec<Box<dyn ToSql>> = vec![];
-
-    // for every filter in the subscription, generate a where clause
-    let mut filter_clauses: Vec<String> = Vec::new();
-    for f in sub.filters.iter() {
-        // individual filter components
+    // individual filter components (single conditions such as an author or event ID)
     let mut filter_components: Vec<String> = Vec::new();
     // Query for "authors", allowing prefix matches
     if let Some(authvec) = &f.authors {
@@ -421,9 +418,6 @@ fn query_from_sub(sub: &Subscription) -> (String, Vec<Box<dyn ToSql>>) {
         let authors_clause = format!("({})", auth_searches.join(" OR "));
         filter_components.push(authors_clause);
     }
-        if let Some(lim) = f.limit {
-            limit = Some(lim)
-        }
     // Query for Kind
     if let Some(ks) = &f.kinds {
         // kind is number, no escaping needed
@@ -495,34 +489,45 @@ fn query_from_sub(sub: &Subscription) -> (String, Vec<Box<dyn ToSql>>) {
         let until_clause = format!("created_at < {}", f.until.unwrap());
         filter_components.push(until_clause);
     }
-
-        // combine all clauses, and add to filter_clauses
-        if !filter_components.is_empty() {
-            let mut fc = "( ".to_owned();
-            fc.push_str(&filter_components.join(" AND "));
-            fc.push_str(" )");
-            filter_clauses.push(fc);
-        }
-    }
-
     // never display hidden events
     query.push_str(" WHERE hidden!=TRUE");
+    // build filter component conditions
+    if !filter_components.is_empty() {
+        query.push_str(" AND ");
+        query.push_str(&filter_components.join(" AND "));
+    }
+    // Apply per-filter limit to this subquery.
+    // The use of a LIMIT implies a DESC order, to capture only the most recent events.
+    if let Some(lim) = f.limit {
+        query.push_str(&format!(" ORDER BY e.created_at DESC LIMIT {}", lim))
+    } else {
+        query.push_str(" ORDER BY e.created_at ASC")
+    }
+    (query, params)
+}
 
-    // combine all filters with OR clauses, if any exist
-    if !filter_clauses.is_empty() {
-        query.push_str(" AND (");
-        query.push_str(&filter_clauses.join(" OR "));
-        query.push_str(") ");
+/// Create a dynamic SQL query string and params from a subscription.
+fn query_from_sub(sub: &Subscription) -> (String, Vec<Box<dyn ToSql>>) {
+    // build a dynamic SQL query for an entire subscription, based on
+    // SQL subqueries for filters.
+    let mut subqueries: Vec<String> = Vec::new();
+    // subquery params
+    let mut params: Vec<Box<dyn ToSql>> = vec![];
+    // for every filter in the subscription, generate a subquery
+    for f in sub.filters.iter() {
+        let (f_subquery, mut f_params) = query_from_filter(&f);
+        subqueries.push(f_subquery);
+        params.append(&mut f_params);
     }
-    // add order clause
-    query.push_str(&format!(
-        " ORDER BY created_at {}",
-        limit.map_or("ASC", |_| "DESC")
-    ));
-    if let Some(lim) = limit {
-        query.push_str(&format!(" LIMIT {}", lim))
-    }
-    debug!("query string: {}", query);
+    // encapsulate subqueries into select statements
+    let subqueries_selects: Vec<String> = subqueries
+        .iter()
+        .map(|s| {
+            return format!("SELECT content, created_at FROM ({})", s);
+        })
+        .collect();
+    let query: String = subqueries_selects.join(" UNION ");
+    info!("final query string: {}", query);
     (query, params)
 }
 

src/subscription.rs

@@ -32,7 +32,7 @@ pub struct ReqFilter {
     /// List of author public keys
     pub authors: Option<Vec<String>>,
     /// Limit number of results
-    pub limit: Option<u32>,
+    pub limit: Option<u64>,
     /// Set of tags
     #[serde(skip)]
     pub tags: Option<HashMap<String, HashSet<String>>>,