# Marten Single Stream Projection

> Create a Marten Single Stream Projection to aggregate events from a single stream into a view (e.g., entity details).

- Skill: `majiayu000/marten-single-stream-projection` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds add majiayu000/marten-single-stream-projection`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/marten-single-stream-projection/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/marten-single-stream-projection

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Follow this guide to create a **Single Stream Projection** in Marten. This projection creates a single document for a single event stream (aggregate instance).

1.  **Define the Projection Class**
    -   Create a `class` in `src/BookStore.ApiService/Projections/`
    -   **Naming**: `{Resource}Projection` (e.g., `AuthorDetailsProjection`)
    -   **Base Class**: `SingleStreamProjection<T>` matches the standard pattern for explicit single stream aggregation.
    -   **Template**: `templates/Projection.cs`

2.  **Configure in Marten**
    -   Open `src/BookStore.ApiService/Infrastructure/Extensions/MartenConfigurationExtensions.cs`
    -   Register the projection in `RegisterProjections`:
        ```csharp
        options.Projections.Add<AuthorDetailsProjection>(ProjectionLifecycle.Async);
        ```
    -   *Note*: `Async` is recommended for performance. `Inline` handles consistency but impacts write performance.

3.  **Indexing (Optional)**
    -   In `ConfigureIndexes`:
        ```csharp
        options.Schema.For<AuthorDetailsProjection>().Index(x => x.Name);
        ```

4.  **Querying**
    -   Query this projection by ID (matching the stream ID) or other fields.
    -   Example: `session.LoadAsync<AuthorDetailsProjection>(streamId)`

## Related Skills
- `/marten__aggregate_scaffold`: Create the events first.
- `/marten__get_by_id`: Create an endpoint to query this projection.
- `/marten__list_query`: Create list endpoints if the projection supports browsing.