Module: Chat

Slack-style messaging: 1:1 DMs, group DMs, named channels, threads, reactions, mentions, pins. Module Order = 800 (after Notifications 750, so Notifications' handlers register first).

Entities / DbContext: ChatChannel (aggregate, soft-deletable) + ChannelMember; Message (aggregate) + MessageAttachment/MessageMention/MessageReaction. ChatDbContext : BaseDbContext, schema chat. Publishes MentionedInChannelIntegrationEvent. Areas: Channels, Messages (incl. pin/edit/delete/threads), Reactions, Search. Full list: Features/v1/ or /scalar.

Gotchas

• EF value-generation for nav children — MessageConfiguration sets Property(x => x.Id).ValueGeneratedNever() for child collections (attachments/mentions/reactions). The domain assigns Guid.CreateVersion7() in factories; without this EF treats nav-collection children as Modified → 0-row UPDATE instead of INSERT. See database.md.
• ChatDbContext calls base.OnModelCreating LAST so tenant auto-apply sees the configured child types.
• ChannelAuthorization (Features/v1/Internal/): RequireMember throws NotFound (404) (not 403) so non-members can't probe channel existence; RequireAdmin throws ForbiddenException. Use these in every channel/message handler.
• SignalR via IHubContext<AppHub> (the shared hub in BuildingBlocks), groups channel:{id}. The hub reads the user via Context.User, not ICurrentUser (see realtime.md). Chat registers IChannelMembershipChecker/IUserChannelLookup adapters so the shared hub can authorize channel groups.
• SendMessage publishes MentionedInChannelIntegrationEvent per distinct mentioned user; Notifications consumes it.
• DMs use a sorted DirectKey ("{lo}:{hi}") for find-or-create; threads are single-level only.
• Chat attachments register ChatChannelFileAccessPolicy (OwnerType "ChatChannel"): attach/read require membership, delete is uploader-only (see modules/files.md).