[ENG-425] Fix template frontmatter bug (#219)

* fix bug

* format

* address PR comments

* address PR comments

Author: trangdoan982

apps/obsidian/src/utils/templates.ts

@@ -1,10 +1,49 @@
-import { App, TFile, TFolder, TAbstractFile } from "obsidian";
+import {
+  App,
+  TFile,
+  TFolder,
+  TAbstractFile,
+  getFrontMatterInfo,
+} from "obsidian";
 
 type TemplatePluginInfo = {
   isEnabled: boolean;
   folderPath: string;
 };
 
+const mergeFrontmatter = (
+  template: Record<string, any>,
+  current: Record<string, any>,
+): Record<string, any> => {
+  const result = { ...template };
+
+  for (const [key, currentValue] of Object.entries(current)) {
+    const templateValue = result[key];
+
+    if (templateValue === undefined) {
+      result[key] = currentValue;
+    } else if (Array.isArray(templateValue) && Array.isArray(currentValue)) {
+      const merged = [...templateValue, ...currentValue];
+      result[key] = [...new Set(merged)];
+    } else if (
+      typeof templateValue === "object" &&
+      templateValue !== null &&
+      typeof currentValue === "object" &&
+      currentValue !== null &&
+      !Array.isArray(templateValue) &&
+      !Array.isArray(currentValue)
+    ) {
+      // Both are objects, merge recursively
+      result[key] = mergeFrontmatter(templateValue, currentValue);
+    } else {
+      // Current value takes precedence for primitives
+      result[key] = currentValue;
+    }
+  }
+
+  return result;
+};
+
 export const getTemplatePluginInfo = (app: App): TemplatePluginInfo => {
   try {
     const templatesPlugin = (app as any).internalPlugins?.plugins?.templates;
@@ -86,11 +125,28 @@ export const applyTemplate = async ({
 
     const templateContent = await app.vault.read(templateFile);
 
-    const currentContent = await app.vault.read(targetFile);
+    const templateFrontmatter =
+      app.metadataCache.getFileCache(templateFile)?.frontmatter || {};
+    const currentFrontmatter =
+      app.metadataCache.getFileCache(targetFile)?.frontmatter || {};
+
+    const mergedFrontmatter = mergeFrontmatter(
+      templateFrontmatter,
+      currentFrontmatter,
+    );
 
-    const newContent = currentContent + templateContent;
+    await app.fileManager.processFrontMatter(targetFile, (fm) => {
+      Object.assign(fm, mergedFrontmatter);
+    });
 
-    await app.vault.modify(targetFile, newContent);
+    const frontmatterInfo = getFrontMatterInfo(templateContent);
+    const templateBody = frontmatterInfo.exists
+      ? templateContent.slice(frontmatterInfo.contentStart)
+      : templateContent;
+
+    if (templateBody.trim()) {
+      await app.vault.append(targetFile, templateBody);
+    }
 
     return true;
   } catch (error) {