api-server/src/modules/rag-chat/rag-chat.service.ts

import { Injectable, NotFoundException, Logger, Optional } from '@nestjs/common';
import { PrismaService } from '../../infrastructure/database/prisma.service';
import { ContentSafetyService } from '../content-safety/content-safety.service';
import { AiGatewayService } from '../ai/gateway/ai-gateway.service';

const MAX_CONTEXT_CHARS = 4000;

@Injectable()
export class RagChatService {
  private readonly logger = new Logger(RagChatService.name);

  constructor(
    private readonly prisma: PrismaService,
    @Optional() private readonly safety?: ContentSafetyService,
    @Optional() private readonly aiGateway?: AiGatewayService,
  ) {}

  async createSession(userId: string, knowledgeBaseId: string, title?: string) {
    return this.prisma.chatSession.create({
      data: { userId, knowledgeBaseId, title: title || '新对话' },
    });
  }

  async listSessions(userId: string, kbId?: string) {
    return this.prisma.chatSession.findMany({
      where: { userId, ...(kbId ? { knowledgeBaseId: kbId } : {}) },
      orderBy: { updatedAt: 'desc' },
    });
  }

  async getMessages(sessionId: string) {
    return this.prisma.chatMessage.findMany({
      where: { sessionId },
      orderBy: { createdAt: 'asc' },
      include: { citations: true },
    });
  }

  async sendMessage(userId: string, sessionId: string, content: string) {
    const session = await this.prisma.chatSession.findUnique({ where: { id: sessionId } });
    if (!session || session.userId !== userId) throw new NotFoundException('对话不存在');

    // Content safety
    const inputCheck = await this.safety?.check(content, { userId, contentType: 'rag_input' });
    if (inputCheck && !inputCheck.safe) {
      return { blocked: true, message: '输入包含违规内容，请修改后重试' };
    }

    // Save user message
    await this.prisma.chatMessage.create({
      data: { sessionId, role: 'user', content },
    });

    // Retrieve knowledge base context
    const context = await this.loadContext(session.knowledgeBaseId);

    // Generate AI response
    let reply: string;
    let citations: any[] = [];

    if (this.aiGateway && context.text) {
      try {
        const messages = [
          { role: 'system' as const, content: this.buildSystemPrompt(context.text) },
          { role: 'user' as const, content },
        ];
        const resp = await this.aiGateway.generate({
          feature: 'rag-chat',
          userId,
          tier: 'primary',
          promptKey: 'rag-chat',
          promptVersion: 'v1',
          messages,
          maxTokens: 2048,
        });
        reply = resp.parsed?.answer ?? String(resp.parsed?.content ?? '抱歉，AI 暂时无法生成回答。');
        citations = context.citations;
      } catch (err: any) {
        this.logger.error('AI Gateway failed, falling back', err?.message);
        reply = this.fallbackReply(context.isEmpty);
      }
    } else {
      reply = this.fallbackReply(context.isEmpty);
    }

    // Save AI message
    const aiMsg = await this.prisma.chatMessage.create({
      data: { sessionId, role: 'ai', content: reply, tokens: reply.length },
    });

    // Save citations
    for (const c of citations.slice(0, 5)) {
      await this.prisma.chatCitation.create({
        data: {
          messageId: aiMsg.id,
          chunkId: c.id,
          sourceTitle: c.title ?? null,
          excerptText: c.text.slice(0, 500),
        },
      });
    }

    // Update session timestamp
    await this.prisma.chatSession.update({ where: { id: sessionId }, data: { updatedAt: new Date() } });

    return { message: aiMsg, citations };
  }

  async deleteSession(sessionId: string) {
    await this.prisma.chatCitation.deleteMany({ where: { message: { sessionId } } });
    await this.prisma.chatMessage.deleteMany({ where: { sessionId } });
    await this.prisma.chatSession.delete({ where: { id: sessionId } });
    return { success: true };
  }

  // ── Private ──

  private async loadContext(kbId: string) {
    try {
      const items = await this.prisma.knowledgeItem.findMany({
        where: { knowledgeBaseId: kbId, deletedAt: null },
        select: { id: true, title: true, content: true, summary: true },
        orderBy: { updatedAt: 'desc' },
        take: 30,
      });

      if (items.length === 0) return { text: '', citations: [], isEmpty: true };

      const parts: string[] = [];
      const citations: any[] = [];

      let total = 0;
      for (const item of items) {
        const t = item.content || item.summary || '';
        if (!t || total >= MAX_CONTEXT_CHARS) break;
        const snippet = t.slice(0, Math.min(t.length, 500));
        parts.push(`【${item.title}】${snippet}`);
        citations.push({ id: item.id, text: snippet, score: 1.0, title: item.title });
        total += snippet.length;
      }

      return { text: parts.join('\n\n'), citations, isEmpty: false };
    } catch {
      return { text: '', citations: [], isEmpty: true };
    }
  }

  private buildSystemPrompt(context: string) {
    return `你是知习 AI 学习助手。基于以下知识库内容回答用户问题，回答应准确、简洁、有依据。

## 知识库内容
${context}

## 回答要求
- 基于提供的知识库内容回答，不要编造信息
- 如果知识库内容不足以回答问题，请诚实告知
- 回答时可以用「根据知识库中的《xxx》...」引用来源`;
  }

  private fallbackReply(isEmpty: boolean) {
    if (isEmpty) {
      return '当前知识库还没有知识点内容。请先上传资料或添加知识点，我就可以基于它们回答你的问题了。';
    }
    return '抱歉，AI 服务暂时不可用，请稍后再试。';
  }
}