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exports.createLesson = async (req, res, next) => {
  try {
    const {
      classId,
      sow,
      grade,
      specificTopic,
      activityType,
      additionalNotes,
    } = req.body;

    // ... [Validation logic skipped for brevity] ...

    const user = await User.findById(req.user.id).select("+geminiApiKey");
    const geminiApiKey = user.getGeminiApiKey();

    const genAI = new GoogleGenerativeAI(geminiApiKey);

    // --- Context & Prompt Building ---
    let promptContext = `
      You are a Malaysia Educator. Your task is to create a lesson plan tally to Scheme of Work (SoW), You may be creative, engaging 60-minute lesson plan for a ${grade} class for the subject: ${subject}.
    `;

    // 1. Material & SOW Logic
    if (materialPart || materialContent) {
      promptContext += `
      IMPORTANT: Base your lesson plan primarily on the provided material (attached document or text context).
      
      ${materialContent ? `--- MATERIAL CONTEXT ---\n${materialContent}\n--- END MATERIAL CONTEXT ---` : ""}
      
      You should still align it with the SoW topic if provided below, but prioritize the material content.
      SOW/Curriculum Context (for alignment only):
      ${JSON.stringify(sow, null, 2)}
      `;
    }
    
    // ... [Prompt parameters construction] ...

    promptContext += `
      The ONLY acceptable format for the 'activities' object is shown in this example (Adapt content for ${subject}):
      {
        "learningObjective": "By the end of the lesson, students will be able to...",
        "successCriteria": [ "I can...", "I can..." ],
        "activities": {
          "preLesson": [ "Teacher introduces...", "Students..." ],
          "duringLesson": [ "Activity 1...", "Activity 2..." ],
          "postLesson": [ "Closure activity..." ]
        }
      }
    `;

    // Call Gemini AI
    const model = genAI.getGenerativeModel({ model: "gemini-2.0-flash" });
    const result = await model.generateContent([promptContext]);
    const response = await result.response;
    const text = response.text();

    // Parse and Validate JSON
    const generatedPlan = JSON.parse(text.replace(/\`\`\`json/g, "").replace(/\`\`\`/g, "").trim());
    
    // Validation using Joi
    const { error, value } = lessonPlanValidationSchema.validate(generatedPlan);
    if (error) throw new Error(`AI response failed validation: ${error.details[0].message}`);

    res.status(200).json({ success: true, data: value });

  } catch (error) {
    console.error("Lesson generation error:", error.message);
    res.status(500).json({ success: false, message: error.message });
  }
};