Add endpoint for querying messages by ID

PluralKit.API/Controllers/MessageController.cs

@@ -0,0 +1,48 @@
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc;
+using Newtonsoft.Json;
+using NodaTime;
+
+namespace PluralKit.API.Controllers
+{
+    public struct MessageReturn
+    {
+        [JsonProperty("timestamp")] public Instant Timestamp;
+        [JsonProperty("id")] public string Id;
+        [JsonProperty("sender")] public string Sender;
+        [JsonProperty("channel")] public string Channel;
+
+        [JsonProperty("system")] public PKSystem System;
+        [JsonProperty("member")] public PKMember Member;
+    }
+    
+    [ApiController]
+    [Route("v1/msg")]
+    [Route("msg")]
+    public class MessageController: ControllerBase
+    {
+        private MessageStore _messages;
+
+        public MessageController(MessageStore messages)
+        {
+            _messages = messages;
+        }
+
+        [HttpGet("{mid}")]
+        public async Task<ActionResult<MessageReturn>> GetMessage(ulong mid)
+        {
+            var msg = await _messages.Get(mid);
+            if (msg == null) return NotFound("Message not found.");
+
+            return new MessageReturn
+            {
+                Timestamp = Instant.FromUnixTimeMilliseconds((long) (msg.Message.Mid >> 22) + 1420070400000),
+                Id = msg.Message.Mid.ToString(),
+                Channel = msg.Message.Channel.ToString(),
+                Sender = msg.Message.Sender.ToString(),
+                Member = msg.Member,
+                System = msg.System
+            };
+        } 
+    }
+}

docs/3-api-documentation.md

@@ -53,6 +53,16 @@ The following three models (usually represented in JSON format) represent the va
 |timestamp|datetime||
 |members|list of id/Member|Is sometimes in plain ID list form (eg. `GET /s/<id>/switches`), sometimes includes the full Member model (eg. `GET /s/<id>/fronters`).|
 
+### Message model
+|Key|Type|Notes|
+|---|---|---
+|timestamp|datetime||
+|id|snowflake|The ID of the message sent by the webhook. Encoded as string for precision reasons.|
+|sender|snowflake|The user ID of the account that triggered the proxy. Encoded as string for precision reasons.|
+|channel|snowflake|The ID of the channel the message was sent in. Encoded as string for precision reasons.|
+|system|full System object|The system that proxied the message.|
+|member|full Member object|The member that proxied the message.|
+
 ## Endpoints
 
 ### GET /s
@@ -298,9 +308,47 @@ Queries a system by its linked Discord account ID (17/18-digit numeric snowflake
 }
 ```
 
+### GET /msg/\<id>
+Looks up a proxied message by its message ID. Returns `404 Not Found` if the message ID is invalid or wasn't found (eg. was deleted or not proxied by PK).
+
+#### Example request
+    GET https://api.pluralkit.me/v1/msg/601014599386398700
+
+#### Example response
+```json
+{
+    "timestamp": "2019-07-17T11:37:26.805Z",
+    "id": "601014599386398700",
+    "sender": "466378653216014359",
+    "channel": "471388251102380000",
+    "system": {
+        "id": "abcde",
+        "name": "My System",
+        "description": "This is my system description. Yay.",
+        "tag": "[MySys]",
+        "avatar_url": "https://path/to/avatar/image.png",
+        "tz": "Europe/Copenhagen",
+        "created": "2019-01-01T14:30:00.987654Z"
+    },
+    "member": {
+        "id": "qwert",
+        "name": "Craig Johnson",
+        "color": "ff7000",
+        "avatar_url": "https://path/to/avatar/image.png",
+        "birthday": "1997-07-14",
+        "pronouns": "he/him or they/them",
+        "description": "I am Craig, example user extraordinaire.",
+        "prefix": "[",
+        "suffix": "]",
+        "created": "2019-01-01T15:00:00.654321Z"
+    }
+}
+```
+
 ## Version history
 * 2019-07-17
   * Add endpoint for querying system by account
+  * Add endpoint for querying message contents
 * 2019-07-10 **(v1)**
   * First specified version
 * (prehistory)