backend/src/models/Message.js

const mongoose = require('mongoose');

const messageSchema = new mongoose.Schema({
  conversationId: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Conversation',
    required: true
  },
  sender: {
    type: String,
    enum: ['user', 'assistant'],
    required: true
  },
  content: {
    type: String,
    required: true,
    maxlength: 10000
  },
  metadata: {
    model: String,
    tokens: Number,
    processingTime: Number
  },
  isEdited: {
    type: Boolean,
    default: false
  },
  editedAt: Date
}, {
  timestamps: true
});

// Index for better query performance
messageSchema.index({ conversationId: 1, createdAt: 1 });

module.exports = mongoose.model('Message', messageSchema);