From 6add25c97d30e3631fce1317b3d3fcc6cca512d0 Mon Sep 17 00:00:00 2001
From: Marco Roth <marco.roth@intergga.ch>
Date: Tue, 11 Aug 2026 18:36:51 +0200
Subject: [PATCH] Language Service: Resolve partials against the view root the
 index found

---
 .../packages/language-server/src/session.ts   | 13 +++++-
 .../src/definition_provider.ts                |  9 +++-
 .../test/definition_provider.test.ts          | 42 +++++++++++++++++++
 3 files changed, 61 insertions(+), 3 deletions(-)

diff --git a/javascript/packages/language-server/src/session.ts b/javascript/packages/language-server/src/session.ts
index 9a7a4c1eb..8bb32be75 100644
--- a/javascript/packages/language-server/src/session.ts
+++ b/javascript/packages/language-server/src/session.ts
@@ -1,3 +1,4 @@
+import { join } from "node:path"
 import { existsSync, readFileSync } from "node:fs"
 import { Connection, InitializeParams } from "vscode-languageserver/node"
 
@@ -57,7 +58,8 @@ export class Session {
         } catch {
           return null
         }
-      }
+      },
+      documentPath => this.viewRootFor(documentPath),
     )
 
     this.projects = new Projects(this.connection, this.workspaceFolders, {
@@ -102,6 +104,15 @@ export class Session {
     })
   }
 
+  private viewRootFor(documentPath: string): string | null {
+    const project = this.projects.containing(documentPath)
+    const viewRoot = project?.partialIndexService.index?.viewRoot
+
+    if (!project || viewRoot === undefined) return null
+
+    return viewRoot === "." ? project.root : join(project.root, viewRoot)
+  }
+
   async init() {
     this.connection.console.log(`[Client] Diagnostic related information: ${this.capabilities.hasDiagnosticRelatedInformation ? "supported" : "not supported"}`)
 
diff --git a/javascript/packages/language-service/src/definition_provider.ts b/javascript/packages/language-service/src/definition_provider.ts
index c9f1db9a7..482719c94 100644
--- a/javascript/packages/language-service/src/definition_provider.ts
+++ b/javascript/packages/language-service/src/definition_provider.ts
@@ -36,15 +36,18 @@ export class DefinitionProvider {
   private parserService: ParserService
   private exists: (filePath: string) => boolean
   private read: (filePath: string) => string | null
+  private viewRootFor: (documentPath: string) => string | null
 
   constructor(
     parserService: ParserService,
     exists: (filePath: string) => boolean,
-    read: (filePath: string) => string | null
+    read: (filePath: string) => string | null,
+    viewRootFor: (documentPath: string) => string | null = () => null
   ) {
     this.parserService = parserService
     this.exists = exists
     this.read = read
+    this.viewRootFor = viewRootFor
   }
 
   getDefinition(document: TextDocument, position: Position): LocationLink[] {
@@ -374,8 +377,10 @@ export class DefinitionProvider {
   }
 
   private viewsRoot(documentPath: string): string | null {
-    const index = documentPath.lastIndexOf(VIEWS_DIRECTORY)
+    const indexed = this.viewRootFor(documentPath)
+    if (indexed !== null) return indexed
 
+    const index = documentPath.lastIndexOf(VIEWS_DIRECTORY)
     if (index === -1) return null
 
     return documentPath.slice(0, index + VIEWS_DIRECTORY.length - 1)
diff --git a/javascript/packages/language-service/test/definition_provider.test.ts b/javascript/packages/language-service/test/definition_provider.test.ts
index 67741b234..eeae6dc1c 100644
--- a/javascript/packages/language-service/test/definition_provider.test.ts
+++ b/javascript/packages/language-service/test/definition_provider.test.ts
@@ -667,4 +667,46 @@ describe("DefinitionProvider", () => {
       expect(definitions(service, content, `"card"`, "file:///project/lib/templates/mailer.html.erb")).toEqual([])
     })
   })
+
+  describe("view root from the index", () => {
+    const TEMPLATES_URI = "file:///project/templates/events/show.html.erb"
+
+    function serviceRootedAt(viewRoot: string | null, ...files: string[]) {
+      const existing = new Set(files)
+
+      return new DefinitionProvider(
+        parserService,
+        filePath => existing.has(filePath),
+        () => "",
+        () => viewRoot,
+      )
+    }
+
+    it("resolves a qualified partial against the root the index reports", () => {
+      const service = serviceRootedAt("/project/templates", "/project/templates/shared/_header.html.erb")
+
+      expect(uris(service, `<%= render "shared/header" %>`, "shared/header", TEMPLATES_URI))
+        .toContain("file:///project/templates/shared/_header.html.erb")
+    })
+
+    it("finds nothing under a conventional layout the project does not use", () => {
+      const service = createService("/project/templates/shared/_header.html.erb")
+
+      expect(uris(service, `<%= render "shared/header" %>`, "shared/header", TEMPLATES_URI)).toEqual([])
+    })
+
+    it("treats a project root view root as the project root itself", () => {
+      const service = serviceRootedAt("/project", "/project/shared/_header.html.erb")
+
+      expect(uris(service, `<%= render "shared/header" %>`, "shared/header", "file:///project/events/show.html.erb"))
+        .toContain("file:///project/shared/_header.html.erb")
+    })
+
+    it("falls back to the conventional layout when nothing is indexed", () => {
+      const service = serviceRootedAt(null, "/project/app/views/shared/_header.html.erb")
+
+      expect(uris(service, `<%= render "shared/header" %>`, "shared/header"))
+        .toContain("file:///project/app/views/shared/_header.html.erb")
+    })
+  })
 })