Add $dateBucketCount support (#10)

* add support for $dateBucketCount

* update readme

Author: haohanyang

README.md

@@ -44,7 +44,7 @@ db.transactions.aggregate({
 });
 ```
 
-You can also use the `"$from"` and `"$to"` conventions originated from a legacy plugin
+You can also use the `"$from"`, `"$to"` and `"$dateBucketCount"`(number of intervals in the time range) conventions originated from a legacy plugin.
 ```json
 [
     {
@@ -57,6 +57,15 @@ You can also use the `"$from"` and `"$to"` conventions originated from a legacy
     }
 ]
 ```
+```json
+{
+    "$bucketAuto": {
+        "groupBy": "$timestamp",
+        "buckets": "$dateBucketCount",
+        // 
+    }
+}
+```
 ### Query type
 #### Time series
 Time series query type is suitable for time series panels. The query results should consist the following fields:

src/datasource.ts

@@ -1,16 +1,9 @@
-import { DataSourceInstanceSettings, CoreApp, ScopedVars, DataQueryRequest, DataQueryResponse, DateTime } from "@grafana/data";
+import { DataSourceInstanceSettings, CoreApp, ScopedVars, DataQueryRequest, DataQueryResponse } from "@grafana/data";
 import { DataSourceWithBackend, getTemplateSrv } from "@grafana/runtime";
-import { parseJsQuery } from "./utils";
+import { parseJsQuery, datetimeToJson, getBucketCount } from "./utils";
 import { MongoQuery, MongoDataSourceOptions, DEFAULT_QUERY, QueryLanguage } from "./types";
 import { Observable } from "rxjs";
 
-function datetimeToJson(datetime: DateTime) {
-  return JSON.stringify({
-    $date: {
-      $numberLong: datetime.toDate().getTime().toString()
-    }
-  });
-}
 
 export class DataSource extends DataSourceWithBackend<MongoQuery, MongoDataSourceOptions> {
   constructor(instanceSettings: DataSourceInstanceSettings<MongoDataSourceOptions>) {
@@ -44,6 +37,7 @@ export class DataSource extends DataSourceWithBackend<MongoQuery, MongoDataSourc
         ...query,
         queryText: queryText.replaceAll(/"\$from"/g, datetimeToJson(request.range.from))
           .replaceAll(/"\$to"/g, datetimeToJson(request.range.to))
+          .replaceAll(/"\$dateBucketCount"/g, getBucketCount(request.range.from, request.range.to, request.intervalMs).toString())
       };
     });
 

src/utils.ts

@@ -1,3 +1,4 @@
+import { DateTime } from "@grafana/data";
 import { JsQueryResult } from "types";
 
 export function validateJsonQueryText(queryText?: string): string | null {
@@ -45,3 +46,23 @@ export function validateJsQueryText(queryText?: string): string | null {
     const { error } = parseJsQuery(queryText);
     return error;
 }
+
+export function datetimeToJson(datetime: DateTime) {
+    return JSON.stringify({
+        $date: {
+            $numberLong: datetime.toDate().getTime().toString()
+        }
+    });
+}
+
+export function getBucketCount(from: DateTime, to: DateTime, intervalMs: number) {
+    let current = from.toDate().getTime();
+    const toMs = to.toDate().getTime();
+    let count = 0;
+    while (current < toMs) {
+        current += intervalMs;
+        count++;
+    }
+
+    return count;
+}