! First pass base implementation for /messages/recent

This api method is made for clients to easily say "give me the n[1,100] latest
messages in some channel
This commit contains code that is largely untested but contains some base code
such that I can correct issues in the next _tested_ commit

json-api/db/src/messages.rs

@@ -203,5 +203,29 @@ impl Message {
 
         Ok(Response::Set(messages))
     }
+
+    /// On success Response::Set<UserMessage>
+    /// On user failure Response::RestrictedInput<String>
+    /// On error Err<SqlError>
+    pub async fn last_n(p: &Pool, limit: i32, channel_id: u64) -> Result<Response<UserMessage>, SqlError> {
+        if limit <= 0 || limit > 100 {
+            return Ok(Response::RestrictedInput("Invalid \"limit\" value".into()))
+        } else {
+            // Reminder: if content_type is not text/plain then content will be empty
+            let q = " SELECT mem.name, msg.id, msg.time, msg.content, msg.content_type, msg.author_id 
+                FROM messages as msg
+                JOIN members as mem ON mem.id = msg.author_id
+                WHERE channel_id = :channel
+                ORDER BY id DESC LIMIT :limit";
+            let params = params!{"limit" => limit, "channel" => channel_id};
+            let mut conn = p.get_conn().await?;
+            let messages = conn.exec_map(q, params, |(name, id, time, content, content_type, author_id)| {
+                UserMessage {
+                    name, id, time, content, content_type, author_id, channel_id
+                }
+            }).await?;
+            return Ok(Response::Set(messages))
+        }
+    }
 }