# CSV Member Import v1 - Implementation Plan

**Version:** 1.0  
**Last Updated:** 2026-01-13  
**Status:** In Progress (Backend Complete, UI Complete, Tests Pending)  
**Related Documents:**
- [Feature Roadmap](./feature-roadmap.md) - Overall feature planning

## Implementation Status

**Completed Issues:**
- ✅ Issue #1: CSV Specification & Static Template Files
- ✅ Issue #2: Import Service Module Skeleton
- ✅ Issue #3: CSV Parsing + Delimiter Auto-Detection + BOM Handling
- ✅ Issue #4: Header Normalization + Per-Header Mapping
- ✅ Issue #5: Validation (Required Fields) + Error Formatting
- ✅ Issue #6: Persistence via Ash Create + Per-Row Error Capture (with Error-Capping)
- ✅ Issue #7: Admin Global Settings LiveView UI (Upload + Start Import + Results + Template Links)
- ✅ Issue #8: Authorization + Limits
- ✅ Issue #11: Custom Field Import (Backend + UI)

**In Progress / Pending:**
- ⏳ Issue #9: End-to-End LiveView Tests + Fixtures
- ⏳ Issue #10: Documentation Polish

**Latest Update:** CSV Import UI fully implemented in GlobalSettingsLive with chunk processing, progress tracking, error display, and custom field support (2026-01-13)

---

## Table of Contents

- [Overview & Scope](#overview--scope)
- [UX Flow](#ux-flow)
- [CSV Specification](#csv-specification)
- [Technical Design Notes](#technical-design-notes)
- [Implementation Issues](#implementation-issues)
- [Rollout & Risks](#rollout--risks)

---

## Overview & Scope

### What We're Building

A **basic CSV member import feature** that allows administrators to upload a CSV file and import new members into the system. This is a **v1 minimal implementation** focused on establishing the import structure without advanced features.

**Core Functionality (v1 Minimal):**
- Upload CSV file via LiveView file upload
- Parse CSV with bilingual header support for core member fields (English/German)
- Auto-detect delimiter (`;` or `,`) using header recognition
- Map CSV columns to core member fields (`first_name`, `last_name`, `email`, `street`, `postal_code`, `city`, `country`)
- **Import custom field values** - Map CSV columns to existing custom fields by name (unknown custom field columns will be ignored with a warning)
- Validate each row (required field: `email`)
- Create members via Ash resource (one-by-one, **no background jobs**, processed in chunks of 200 rows via LiveView messages)
- Display import results: success count, error count, and error details
- Provide static CSV templates (EN/DE)

**Key Constraints (v1):**
- ✅ **Admin-only feature**
- ✅ **No upsert** (create only)
- ✅ **No deduplication** (duplicate emails fail and show as errors)
- ✅ **No mapping wizard** (fixed header mapping via bilingual variants)
- ✅ **No background jobs** (progress via LiveView `handle_info`)
- ✅ **Best-effort import** (row-by-row, no rollback)
- ✅ **UI-only error display** (no error CSV export)
- ✅ **Safety limits** (10 MB, 1,000 rows, chunks of 200)

### Out of Scope (v1)

**Deferred to Future Versions:**
- ❌ Upsert/update existing members
- ❌ Advanced deduplication strategies
- ❌ Column mapping wizard UI
- ❌ Background job processing (Oban/GenStage)
- ❌ Transactional all-or-nothing import
- ❌ Error CSV export/download
- ❌ Batch validation preview before import
- ❌ Dynamic template generation
- ❌ Import history/audit log
- ❌ Import templates for other entities

---

## UX Flow

### Access & Location

**Entry Point:**
- **Location:** Global Settings page (`/settings`)
- **UI Element:** New section "Import Members (CSV)" below "Custom Fields" section
- **Access Control:** Admin-only (enforced at LiveView event level, not entire `/settings` route)

### User Journey

1. **Navigate to Global Settings**
2. **Access Import Section**
   - **Important notice:** Custom fields should be created in Mila before importing CSV files with custom field columns (unknown columns will be ignored with a warning)
   - Upload area (drag & drop or file picker)
   - Template download links (English / German)
   - Help text explaining CSV format and custom field requirements
3. **Ensure Custom Fields Exist (if importing custom fields)**
   - Navigate to Custom Fields section and create required custom fields
   - Note the name/identifier for each custom field (used as CSV header)
4. **Download Template (Optional)**
5. **Prepare CSV File**
   - Include custom field columns using the custom field name as header (e.g., `membership_number`, `birth_date`)
6. **Upload CSV**
7. **Start Import**
   - Runs server-side via LiveView messages (may take up to ~30 seconds for large files)
   - Warning messages if custom field columns reference non-existent custom fields (columns will be ignored)
8. **View Results**
   - Success count
   - Error count
   - First 50 errors, each with:
     - **CSV line number** (header is line 1, first data record begins at line 2)
     - Error message
     - Field name (if applicable)

### Error Handling

- **File too large:** Flash error before upload starts
- **Too many rows:** Flash error before import starts
- **Invalid CSV format:** Error shown in results
- **Partial success:** Results show both success and error counts

---

## CSV Specification

### Delimiter

**Recommended:** Semicolon (`;`)  
**Supported:** `;` and `,`

**Auto-Detection (Header Recognition):**
- Remove UTF-8 BOM *first*
- Extract header record and try parsing with both delimiters
- For each delimiter, count how many recognized headers are present (via normalized variants)
- Choose delimiter with higher recognition; prefer `;` if tied
- If neither yields recognized headers, default to `;`

### Quoting Rules

- Fields may be quoted with double quotes (`"`)
- Escaped quotes: `""` inside quoted field represents a single `"`
- **v1 assumption:** CSV records do **not** contain embedded newlines inside quoted fields. (If they do, parsing may fail or line numbers may be inaccurate.)

### Column Headers

**v1 Supported Fields:**

**Core Member Fields (all importable):**
- `email` / `E-Mail` (required)
- `first_name` / `Vorname` (optional)
- `last_name` / `Nachname` (optional)
- `join_date` / `Beitrittsdatum` (optional, ISO-8601 date)
- `exit_date` / `Austrittsdatum` (optional, ISO-8601 date)
- `notes` / `Notizen` (optional)
- `country` / `Land` / `Staat` (optional)
- `city` / `Stadt` (optional)
- `street` / `Straße` (optional)
- `house_number` / `Hausnummer` / `Nr.` (optional)
- `postal_code` / `PLZ` / `Postleitzahl` (optional)
- `membership_fee_start_date` / `Beitragsbeginn` (optional, ISO-8601 date)

Address column order in import/export matches the members overview: country, city, street, house number, postal code.

**Not supported for import (by design):**
- **membership_fee_status** – Computed field (from fee cycles). Not stored; export-only.
- **groups** – Many-to-many relationship. Would require resolving group names to IDs; not in current scope.
- **membership_fee_type_id** – Foreign key; could be added later (e.g. resolve type name to ID).

**Custom Fields:**
- Any custom field column using the custom field's **name** as the header (e.g., `membership_number`, `birth_date`)
- **Important:** Custom fields must be created in Mila before importing. The CSV header must match the custom field name exactly (same normalization as member fields).
- **Behavior:** If the CSV contains custom field columns that don't exist in Mila, a warning message will be shown and those columns will be ignored during import.
- **Value Validation:** Custom field values are validated according to the custom field type:
  - **string**: Any text value (trimmed)
  - **integer**: Must be a valid integer (e.g., `42`, `-10`). Invalid values will cause a row error with the custom field name and reason.
  - **boolean**: Accepts `true`, `false`, `1`, `0`, `yes`, `no`, `ja`, `nein` (case-insensitive). Invalid values will cause a row error.
  - **date**: Must be in ISO-8601 format (YYYY-MM-DD, e.g., `2024-01-15`). Invalid values will cause a row error.
  - **email**: Must be a valid email format (contains `@`, 5-254 characters, valid format). Invalid values will cause a row error.
- **Error Messages:** Custom field validation errors are included in the import error list with format: `custom_field: <name> – <reason>` (e.g., `custom_field: Alter – expected integer, got: abc`)

**Member Field Header Mapping:**

| Canonical Field | English Variants | German Variants |
|---|---|---|
| `first_name` | `first_name`, `firstname` | `Vorname`, `vorname` |
| `last_name` | `last_name`, `lastname`, `surname` | `Nachname`, `nachname`, `Familienname` |
| `email` | `email`, `e-mail`, `e_mail` | `E-Mail`, `e-mail`, `e_mail` |
| `join_date` | `join date`, `join_date` | `Beitrittsdatum`, `beitritts-datum` |
| `exit_date` | `exit date`, `exit_date` | `Austrittsdatum`, `austritts-datum` |
| `notes` | `notes` | `Notizen`, `bemerkungen` |
| `street` | `street`, `address` | `Straße`, `strasse`, `Strasse` |
| `house_number` | `house number`, `house_number`, `house no` | `Hausnummer`, `Nr`, `Nr.`, `Nummer` |
| `postal_code` | `postal_code`, `zip`, `postcode` | `PLZ`, `plz`, `Postleitzahl`, `postleitzahl` |
| `city` | `city`, `town` | `Stadt`, `stadt`, `Ort` |
| `country` | `country` | `Land`, `land`, `Staat`, `staat` |
| `membership_fee_start_date` | `membership fee start date`, `membership_fee_start_date`, `fee start` | `Beitragsbeginn`, `beitrags-beginn` |

**Header Normalization (used consistently for both input headers AND mapping variants):**
- Trim whitespace
- Convert to lowercase
- Normalize Unicode: `ß` → `ss` (e.g., `Straße` → `strasse`)
- Replace hyphens/whitespace with underscores: `E-Mail` → `e_mail`, `phone number` → `phone_number`
- Collapse multiple underscores: `e__mail` → `e_mail`
- Case-insensitive matching

**Unknown columns:** ignored (no error)

**Required fields:** `email`

**Custom Field Columns:**
- Custom field columns are identified by matching the normalized CSV header to the custom field `name` (not slug)
- Same normalization rules apply as for member fields (trim, lowercase, Unicode normalization, underscore replacement)
- Unknown custom field columns (non-existent names) will be ignored with a warning message

### CSV Template Files

**Location:**
- `priv/static/templates/member_import_en.csv`
- `priv/static/templates/member_import_de.csv`

**Content:**
- Header row with required + common optional fields
- **Note:** Custom field columns are not included in templates by default (users add them based on their custom field configuration)
- One example row
- Uses semicolon delimiter (`;`)
- UTF-8 encoding **with BOM** (Excel compatibility)

**Template Access:**
- Templates are static files in `priv/static/templates/`
- Served at:
  - `/templates/member_import_en.csv`
  - `/templates/member_import_de.csv`
- In LiveView, link them using Phoenix static path helpers (e.g. `~p` or `Routes.static_path/2`, depending on Phoenix version).

**Example Usage in LiveView Templates:**

```heex
<!-- Using ~p sigil (Phoenix 1.7+) -->
<.link href={~p"/templates/member_import_en.csv"} download>
  <%= gettext("Download English Template") %>
</.link>

<.link href={~p"/templates/member_import_de.csv"} download>
  <%= gettext("Download German Template") %>
</.link>

<!-- Alternative: Using Routes.static_path/2 -->
<.link href={Routes.static_path(MvWeb.Endpoint, "/templates/member_import_en.csv")} download>
  <%= gettext("Download English Template") %>
</.link>
```

**Note:** The `templates` directory must be included in `MvWeb.static_paths()` (configured in `lib/mv_web.ex`) for the files to be served.

### File Limits

- **Max file size:** 10 MB
- **Max rows:** 1,000 rows (excluding header)
- **Processing:** chunks of 200 (via LiveView messages)
- **Encoding:** UTF-8 (BOM handled)

---

## Technical Design Notes

### Architecture Overview

```
┌─────────────────┐
│  LiveView UI    │  (GlobalSettingsLive or component)
│  - Upload area  │
│  - Progress     │
│  - Results      │
└────────┬────────┘
         │ prepare
         ▼
┌─────────────────────────────┐
│ Import Service              │  (Mv.Membership.Import.MemberCSV)
│ - parse + map + limit checks│  -> returns import_state
│ - process_chunk(chunk)      │  -> returns chunk results
└────────┬────────────────────┘
         │ create
         ▼
┌─────────────────┐
│  Ash Resource   │  (Mv.Membership.Member)
│  - Create       │
└─────────────────┘
```

### Technology Stack

- **Phoenix LiveView:** file upload via `allow_upload/3`
- **NimbleCSV:** CSV parsing (add explicit dependency if missing)
- **Ash Resource:** member creation via `Membership.create_member/1`
- **Gettext:** bilingual UI/error messages

### Module Structure

**New Modules:**
- `lib/mv/membership/import/member_csv.ex` - import orchestration + chunk processing + custom field handling
- `lib/mv/membership/import/csv_parser.ex` - delimiter detection + parsing + BOM handling
- `lib/mv/membership/import/header_mapper.ex` - normalization + header mapping (core fields + custom fields)

**Modified Modules:**
- `lib/mv_web/live/global_settings_live.ex` - render import section, handle upload/events/messages

### Data Flow

1. **Upload:** LiveView receives file via `allow_upload`
2. **Consume:** `consume_uploaded_entries/3` reads file content
3. **Prepare:** `MemberCSV.prepare/2`
   - Strip BOM
   - Detect delimiter (header recognition)
   - Parse header + rows
   - Map headers to canonical fields (core member fields)
   - **Query existing custom fields and map custom field columns by name** (using same normalization as member fields)
   - **Warn about unknown custom field columns** (non-existent names will be ignored with warning)
   - Early abort if required headers missing
   - Row count check
   - Return `import_state` containing chunks, column_map, and custom_field_map
4. **Process:** LiveView drives chunk processing via `handle_info`
   - For each chunk: validate + create member + create custom field values + collect errors
5. **Results:** LiveView shows progress + final summary

### Types & Key Consistency

- **Raw CSV parsing:** returns headers as list of strings, and rows **with csv line numbers**
- **Header mapping:** operates on normalized strings; mapping table variants are normalized once
- **Ash attrs:** built as atom-keyed map (`%{first_name: ..., ...}`)

### Error Model

```elixir
%{
  csv_line_number: 5,       # physical line number in the CSV file
  field: :email,            # optional
  message: "is not a valid email"
}
```

### CSV Line Numbers (Important)

To keep error reporting user-friendly and accurate, **row errors must reference the physical line number in the original file**, even if empty lines are skipped.

**Design decision:** the parser returns rows as:

```elixir
rows :: [{csv_line_number :: pos_integer(), row_map :: map()}]
```

Downstream logic must **not** recompute line numbers from row indexes.

### Authorization

**Enforcement points:**
1. **LiveView event level:** check admin permission in `handle_event("start_import", ...)`
2. **UI level:** render import section only for admin users
3. **Static templates:** public assets (no authorization needed)

Use `Mv.Authorization.PermissionSets` (preferred) instead of hard-coded string checks where possible.

### Safety Limits

- File size enforced by `allow_upload` (`max_file_size`)
- Row count enforced in `MemberCSV.prepare/2` before processing starts
- Chunking is done via **LiveView `handle_info` loop** (sequential, cooperative scheduling)

---

## Implementation Issues

### Issue #1: CSV Specification & Static Template Files

**Dependencies:** None

**Status:** ✅ **COMPLETED**

**Goal:** Define CSV contract and add static templates.

**Tasks:**
- [x] Finalize header mapping variants
- [x] Document normalization rules
- [x] Document delimiter detection strategy
- [x] Create templates in `priv/static/templates/` (UTF-8 with BOM)
  - `member_import_en.csv` with English headers
  - `member_import_de.csv` with German headers
- [x] Document template URLs and how to link them from LiveView
- [x] Document line number semantics (physical CSV line numbers)
- [x] Templates included in `MvWeb.static_paths()` configuration

**Definition of Done:**
- [x] Templates open cleanly in Excel/LibreOffice
- [x] CSV spec section complete

---

### Issue #2: Import Service Module Skeleton

**Dependencies:** None

**Status:** ✅ **COMPLETED**

**Goal:** Create service API and error types.

**API (recommended):**
- `prepare/2` — parse + map + limit checks, returns import_state
- `process_chunk/4` — process one chunk (pure-ish), returns per-chunk results

**Tasks:**
- [x] Create `lib/mv/membership/import/member_csv.ex`
- [x] Define public function: `prepare/2 (file_content, opts \\ [])`
- [x] Define public function: `process_chunk/4 (chunk_rows_with_lines, column_map, custom_field_map, opts \\ [])`
- [x] Define error struct: `%MemberCSV.Error{csv_line_number: integer, field: atom | nil, message: String.t}`
- [x] Document module + API

---

### Issue #3: CSV Parsing + Delimiter Auto-Detection + BOM Handling

**Dependencies:** Issue #2

**Status:** ✅ **COMPLETED**

**Goal:** Parse CSV robustly with correct delimiter detection and BOM handling.

**Tasks:**
- [x] Verify/add NimbleCSV dependency (`{:nimble_csv, "~> 1.0"}`)
- [x] Create `lib/mv/membership/import/csv_parser.ex`
- [x] Implement `strip_bom/1` and apply it **before** any header handling
- [x] Handle `\r\n` and `\n` line endings (trim `\r` on header record)
- [x] Detect delimiter via header recognition (try `;` and `,`)
- [x] Parse CSV and return:
  - `headers :: [String.t()]`
  - `rows :: [{csv_line_number, [String.t()]}]` with correct physical line numbers
- [x] Skip completely empty records (but preserve correct physical line numbers)
- [x] Return `{:ok, headers, rows}` or `{:error, reason}`

**Definition of Done:**
- [x] BOM handling works (Excel exports)
- [x] Delimiter detection works reliably
- [x] Rows carry correct `csv_line_number`

---

### Issue #4: Header Normalization + Per-Header Mapping (No Language Detection)

**Dependencies:** Issue #3

**Status:** ✅ **COMPLETED**

**Goal:** Map each header individually to canonical fields (normalized comparison).

**Tasks:**
- [x] Create `lib/mv/membership/import/header_mapper.ex`
- [x] Implement `normalize_header/1`
- [x] Normalize mapping variants once and compare normalized strings
- [x] Build `column_map` (canonical field -> column index)
- [x] **Early abort if required headers missing** (`email`)
- [x] Ignore unknown columns (member fields only)
- [x] **Separate custom field column detection** (by name, with normalization)

**Definition of Done:**
- [x] English/German headers map correctly
- [x] Missing required columns fails fast

---

### Issue #5: Validation (Required Fields) + Error Formatting

**Dependencies:** Issue #4

**Status:** ✅ **COMPLETED**

**Goal:** Validate each row and return structured, translatable errors.

**Tasks:**
- [x] Implement `validate_row/3 (row_map, csv_line_number, opts)`
- [x] Required field presence (`email`)
- [x] Email format validation (EctoCommons.EmailValidator)
- [x] Trim values before validation
- [x] Gettext-backed error messages

---

### Issue #6: Persistence via Ash Create + Per-Row Error Capture (Chunked Processing)

**Dependencies:** Issue #5

**Status:** ✅ **COMPLETED**

**Goal:** Create members and capture errors per row with correct CSV line numbers.

**Tasks:**
- [x] Implement `process_chunk/4` in service:
  - Input: `[{csv_line_number, row_map}]`
  - Validate + create sequentially
  - Collect counts + first 50 errors (per import overall; LiveView enforces cap across chunks)
  - **Error-Capping:** Supports `existing_error_count` and `max_errors` in opts (default: 50)
  - **Error-Capping:** Only collects errors if under limit, but continues processing all rows
  - **Error-Capping:** `failed` count is always accurate, even when errors are capped
- [x] Implement Ash error formatter helper:
  - Convert `Ash.Error.Invalid` into `%MemberCSV.Error{}`
  - Prefer field-level errors where possible (attach `field` atom)
  - Handle unique email constraint error as user-friendly message
- [x] Map row_map to Ash attrs (`%{first_name: ..., ...}`)
- [x] Custom field value processing and creation

**Important:** **Do not recompute line numbers** in this layer—use the ones provided by the parser.

**Implementation Notes:**
- `process_chunk/4` accepts `opts` with `existing_error_count` and `max_errors` for error capping across chunks
- Error capping respects the limit per import overall (not per chunk)
- Processing continues even after error limit is reached (for accurate counts)

---

### Issue #7: Admin Global Settings LiveView UI (Upload + Start Import + Results + Template Links)

**Dependencies:** Issue #6

**Status:** ✅ **COMPLETED**

**Goal:** UI section with upload, progress, results, and template links.

**Tasks:**
- [x] Render import section only for admins
- [x] **Add prominent UI notice about custom fields:**
  - Display alert/info box: "Custom fields must be created in Mila before importing CSV files with custom field columns"
  - Explain: "Use the custom field name as the CSV column header (same normalization as member fields applies)"
  - Add link to custom fields management section
- [x] Configure `allow_upload/3`:
  - `.csv` only, `max_entries: 1`, `max_file_size: 10MB`, `auto_upload: true` (auto-upload enabled for better UX)
- [x] `handle_event("start_import", ...)`:
  - Admin permission check
  - Consume upload -> read file content
  - Call `MemberCSV.prepare/2`
  - Store `import_state` in assigns (chunks + column_map + metadata)
  - Initialize progress assigns
  - `send(self(), {:process_chunk, 0})`
- [x] `handle_info({:process_chunk, idx}, socket)`:
  - Fetch chunk from `import_state`
  - Call `MemberCSV.process_chunk/4` with error capping support
  - Merge counts/errors into progress assigns (cap errors at 50 overall)
  - Schedule next chunk (or finish and show results)
  - Async task processing with SQL sandbox support for tests
- [x] Results UI:
  - Success count
  - Failure count
  - Error list (line number + message + field)
  - **Warning messages for unknown custom field columns** (non-existent names) shown in results
  - Progress indicator during import
  - Error truncation notice when errors exceed limit

**Template links:**
- [x] Link `/templates/member_import_en.csv` and `/templates/member_import_de.csv` via Phoenix static path helpers.

**Definition of Done:**
- [x] Upload area with drag & drop support
- [x] Template download links (EN/DE)
- [x] Progress tracking during import
- [x] Results display with success/error counts
- [x] Error list with line numbers and field information
- [x] Warning display for unknown custom field columns
- [x] Admin-only access control
- [x] Async chunk processing with proper error handling

---

### Issue #8: Authorization + Limits

**Dependencies:** None (can be parallelized)

**Status:** ✅ **COMPLETED**

**Goal:** Ensure admin-only access and enforce limits.

**Tasks:**
- [x] Admin check in start import event handler (via `Authorization.can?/3`)
- [x] File size enforced in upload config (`max_file_size: 10MB`)
- [x] Row limit enforced in `MemberCSV.prepare/2` (max_rows: 1000, configurable via opts)
- [x] Chunk size limit (200 rows per chunk)
- [x] Error limit (50 errors per import)
- [x] UI-level authorization check (import section only visible to admins)
- [x] Event-level authorization check (prevents unauthorized import attempts)

**Implementation Notes:**
- File size limit: 10 MB (10,485,760 bytes) enforced via `allow_upload/3`
- Row limit: 1,000 rows (excluding header) enforced in `MemberCSV.prepare/2`
- Chunk size: 200 rows per chunk (configurable via opts)
- Error limit: 50 errors per import (configurable via `@max_errors`)
- Authorization uses `MvWeb.Authorization.can?/3` with `:create` permission on `Mv.Membership.Member`

**Definition of Done:**
- [x] Admin-only access enforced at UI and event level
- [x] File size limit enforced
- [x] Row count limit enforced
- [x] Chunk processing with size limits
- [x] Error capping implemented

---

### Issue #9: End-to-End LiveView Tests + Fixtures

**Dependencies:** Issue #7 and #8

**Tasks:**
- [ ] Fixtures:
  - valid EN/DE (core fields only)
  - valid with custom fields
  - invalid
  - unknown custom field name (non-existent, should show warning)
  - too many rows (1,001)
  - BOM + `;` delimiter fixture
  - fixture with empty line(s) to validate correct line numbers
- [ ] LiveView tests:
  - admin sees section, non-admin does not
  - upload + start import
  - success + error rendering
  - row limit + file size errors
  - custom field import success
  - custom field import warning (non-existent name, column ignored)

---

### Issue #10: Documentation Polish (Inline Help Text + Docs)

**Dependencies:** Issue #9

**Tasks:**
- [ ] UI help text + translations
- [ ] CHANGELOG entry
- [ ] Ensure moduledocs/docs

---

### Issue #11: Custom Field Import

**Dependencies:** Issue #6 (Persistence)

**Priority:** High (Core v1 Feature)

**Status:** ✅ **COMPLETED** (Backend + UI Implementation)

**Goal:** Support importing custom field values from CSV columns. Custom fields should exist in Mila before import for best results.

**Important Requirements:**
- **Custom fields should be created in Mila first** - Unknown custom field columns will be ignored with a warning message
- CSV headers for custom fields must match the custom field **name** exactly (same normalization as member fields applies)
- Custom field values are validated according to the custom field type (string, integer, boolean, date, email)
- Unknown custom field columns (non-existent names) will be ignored with a warning - import continues

**Tasks:**
- [x] Extend `header_mapper.ex` to detect custom field columns by name (using same normalization as member fields)
- [x] Query existing custom fields during `prepare/2` to map custom field columns
- [x] Collect unknown custom field columns and add warning messages (don't fail import)
- [x] Map custom field CSV values to `CustomFieldValue` creation in `process_chunk/4`
- [x] Handle custom field type validation (string, integer, boolean, date, email) with proper error messages
- [x] Create `CustomFieldValue` records linked to members during import
- [x] Validate custom field values and return structured errors with custom field name and reason
- [x] UI help text and link to custom field management (implemented in Issue #7)
- [x] Update error messages to include custom field validation errors (format: `custom_field: <name> – expected <type>, got: <value>`)
- [x] Add UI help text explaining custom field requirements (completed in Issue #7):
  - "Custom fields must be created in Mila before importing"
  - "Use the custom field name as the CSV column header (same normalization as member fields)"
  - Link to custom fields management section
- [x] Update CSV templates documentation to explain custom field columns (documented in Issue #1)
- [x] Add tests for custom field import (valid, invalid name, type validation, warning for unknown)

**Definition of Done:**
- [x] Custom field columns are recognized by name (with normalization)
- [x] Warning messages shown for unknown custom field columns (import continues)
- [x] Custom field values are created and linked to members
- [x] Type validation works for all custom field types (string, integer, boolean, date, email)
- [x] UI clearly explains custom field requirements (completed in Issue #7)
- [x] Tests cover custom field import scenarios (including warning for unknown names)
- [x] Error messages include custom field validation errors with proper formatting

**Implementation Notes:**
- Custom field lookup is built in `prepare/2` and passed via `custom_field_lookup` in opts
- Custom field values are formatted according to type in `format_custom_field_value/2`
- Unknown custom field columns generate warnings in `import_state.warnings`

---

## Rollout & Risks

### Rollout Strategy
- Dev → Staging → Production (with anonymized real-world CSV tests)

### Risks & Mitigations

| Risk | Impact | Likelihood | Mitigation |
|---|---:|---:|---|
| Large import timeout | High | Medium | 10 MB + 1,000 rows, chunking via `handle_info` |
| Encoding issues | Medium | Medium | BOM stripping, templates with BOM |
| Invalid CSV format | Medium | High | Clear errors + templates |
| Duplicate emails | Low | High | Ash constraint error -> user-friendly message |
| Performance (no background jobs) | Medium | Low | Small limits, sequential chunk processing |
| Admin access bypass | High | Low | Event-level auth + UI hiding |
| Data corruption | High | Low | Per-row validation + best-effort |

---

## Appendix

### Module File Structure

```
lib/
├── mv/
│   └── membership/
│       └── import/
│           ├── member_csv.ex          # prepare + process_chunk
│           ├── import_runner.ex       # orchestration: file read, progress merge, chunk process, error format
│           ├── csv_parser.ex          # delimiter detection + parsing + BOM handling
│           └── header_mapper.ex       # normalization + header mapping
└── mv_web/
    └── live/
        ├── import_export_live.ex       # mount / handle_event / handle_info + glue only
        └── import_export_live/
            └── components.ex          # UI: custom_fields_notice, template_links, import_form, import_progress, import_results

priv/
└── static/
    └── templates/
        ├── member_import_en.csv
        └── member_import_de.csv

test/
├── mv/
│   └── membership/
│       └── import/
│           ├── member_csv_test.exs
│           ├── csv_parser_test.exs
│           └── header_mapper_test.exs
└── fixtures/
    ├── member_import_en.csv
    ├── member_import_de.csv
    ├── member_import_invalid.csv
    ├── member_import_large.csv
    └── member_import_empty_lines.csv
```

### Example Usage (LiveView)

```elixir
def handle_event("start_import", _params, socket) do
  assert_admin!(socket.assigns.current_user)

  [{_name, content}] =
    consume_uploaded_entries(socket, :csv_file, fn %{path: path}, _entry ->
      {:ok, File.read!(path)}
    end)

  case Mv.Membership.Import.MemberCSV.prepare(content) do
    {:ok, import_state} ->
      socket =
        socket
        |> assign(:import_state, import_state)
        |> assign(:import_progress, %{processed: 0, inserted: 0, failed: 0, errors: []})
        |> assign(:importing?, true)

      send(self(), {:process_chunk, 0})
      {:noreply, socket}

    {:error, reason} ->
      {:noreply, put_flash(socket, :error, reason)}
  end
end

def handle_info({:process_chunk, idx}, socket) do
  %{chunks: chunks, column_map: column_map} = socket.assigns.import_state

  case Enum.at(chunks, idx) do
    nil ->
      {:noreply, assign(socket, importing?: false)}

    chunk_rows_with_lines ->
      {:ok, chunk_result} =
        Mv.Membership.Import.MemberCSV.process_chunk(chunk_rows_with_lines, column_map)

      socket = merge_progress(socket, chunk_result) # caps errors at 50 overall

      send(self(), {:process_chunk, idx + 1})
      {:noreply, socket}
  end
end
```

---

**End of Implementation Plan**
