Special fieldName for keys

This diff introduces a special fieldName when we want to create an index that involves the key. For example, imagine a collection where the keys are `K`, and the values are `{field1: V1, field2: V2}` and we want to create an index `V2:K`, the way we would do it would be with:

```
.map(IndexMapper, 'field2', '$key')
```

The `$key` is treated as a special keyword that will be interpreted as
the key.  Of course, the unfortunate scenario, is if one of the fields
is actually named '$key', in which case the user will have to roll out
their own index, or apply some kind of transformation before the map,
for example:

```
.map(TranformDollarKeyFieldToSomethingElse)
.map(IndexMapper, 'field2', 'theNewNameOfTheDollarKeyField'
```

Author: pikatchu

skipruntime-ts/examples/skipsocial-client.ts

@@ -91,8 +91,10 @@ await fetchJSON(
 );
 
 const friends2 = await fetchJSON(`${url}/friend/${userID1}`, "GET", header);
+const areFriends = await fetchJSON(`${url}/friend-index/${userID1}:${userID2}`, "GET", header);
 
 console.log(friends2);
+console.log(areFriends);
 
 /*****************************************************************************/
 /* Testing messages */

skipruntime-ts/examples/skipsocial-server.ts

@@ -376,6 +376,7 @@ createPUT("friend-request", "friendRequests", async (runtime, id, object) => {
 /*****************************************************************************/
 
 createGET("friend", "friends");
+createGET("friend-index", "friendIndex");
 
 /*****************************************************************************/
 /* Messages */
@@ -470,7 +471,7 @@ createPUT("group-member", "groupMembers", async (runtime, id, object) => {
   await checkIsNotTakenID({
     runtime,
     desc: "The groupID:memberID",
-    resource: "groupMemberPairs",
+    resource: "groupMemberIndex",
     id: groupID + ":" + userID,
   });
 });

skipruntime-ts/examples/skipsocial-utils.ts

@@ -16,13 +16,15 @@ export function makeID<T>(x: T): ID<T> {
   return JSON.stringify(x);
 }
 
-export class IndexMapper implements Mapper<string, TJSON, string, boolean> {
+const keyName: string = "$key";
+
+export class IndexMapper implements Mapper<TJSON, TJSON, string, boolean> {
   private fieldNames: string[];
   constructor(...fieldNames: string[]) {
     this.fieldNames = fieldNames;
   }
   mapElement(
-    _key: string,
+    key: TJSON,
     values: NonEmptyIterator<unknown>,
   ): Iterable<[string, boolean]> {
     let result: Array<[string, boolean]> = [];
@@ -35,6 +37,26 @@ export class IndexMapper implements Mapper<string, TJSON, string, boolean> {
       }
       const fieldValues = [];
       for (const fieldName of this.fieldNames) {
+        if(fieldName === keyName) {
+          if (typeof key !== "string") {
+            throw new Error(
+              "Object field is not a string " +
+                fieldName +
+                ": " +
+                JSON.stringify(value),
+            );
+          }
+          if (key.includes(":")) {
+            throw new Error(
+              "Malformed key (it contains a ':')" +
+                fieldName +
+                ": " +
+                JSON.stringify(value),
+            );
+          }
+          fieldValues.push(key);
+          continue;
+        } 
         if (!(fieldName in value)) {
           throw new Error(
             "Object does not define the field " +

skipruntime-ts/examples/skipsocial.ts

@@ -11,7 +11,6 @@ import type {
 import { runService } from "@skipruntime/server";
 import {
   type ID,
-  makeID,
   makeRequestCollection,
   IndexMapper,
 } from "./skipsocial-utils.js";
@@ -161,21 +160,6 @@ class FriendIndexResource implements Resource {
   }
 }
 
-class FriendIndexMapper implements Mapper<ID<User>, Friend, string, boolean> {
-  mapElement(
-    userID: ID<User>,
-    friends: NonEmptyIterator<Friend>,
-  ): Iterable<[ID<[User, User]>, boolean]> {
-    let result: Array<[string, boolean]> = [];
-    for (const friend of friends) {
-      if (userID < friend.friendID) {
-        result.push([makeID([userID, friend.friendID]), true]);
-      }
-    }
-    return result;
-  }
-}
-
 /*****************************************************************************/
 // Messages and channels
 /*****************************************************************************/
@@ -407,7 +391,7 @@ class Service implements SkipService {
     const friendRequests = inputCollections.friendRequests;
     const friendRequestsIndex = friendRequests.map(IndexMapper, "from", "to");
     const friends = makeRequestCollection(friendRequests).map(FriendMapper);
-    const friendIndex = friends.map(FriendIndexMapper);
+    const friendIndex = friends.map(IndexMapper, "$key", 'friendID');
     const groupMemberIndex = inputCollections.groupMembers.map(
       IndexMapper,
       "groupID",