📝 document record transformation apis

docs/api/record-with-fields.md

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+---
+name: record-with-fields
+group: api
+category: record
+update-time: 20260613
+description: Return a copy of a record with a replaced field array.
+key-word:
+    - record
+    - fields
+    - transform
+    - public
+---
+
+## Record-with-fields
+
+Return a copy of a `Record` with a different field array. This helper is the narrowest way to replace structured metadata while preserving the record's level, message, timestamp, and target.
+
+### Interface
+
+```moonbit
+pub fn Record::with_fields(self : Record, fields : Array[Field]) -> Record {}
+```
+
+#### input
+
+- `self : Record` - Base record whose field array should be replaced.
+- `fields : Array[Field]` - New structured field list for the returned record.
+
+#### output
+
+- `Record` - New record value carrying the updated fields.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This helper returns a new record value instead of mutating the original one.
+- Only the `fields` array is replaced.
+- `level`, `message`, `timestamp_ms`, and `target` are preserved.
+- This is useful when one record should swap its metadata wholesale rather than append more fields.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need To Replace Metadata Entirely
+
+When one record should keep its message and severity but use a new field set:
+```moonbit
+let rec = Record::new(Level::Info, "accepted", fields=[field("user", "alice")])
+let sanitized = rec.with_fields([field("user", "***")])
+```
+
+In this example, the original field array is replaced rather than extended.
+
+#### When Need To Clear Per-record Fields
+
+When structured metadata should be removed from one value:
+```moonbit
+let rec = Record::new(Level::Warn, "retry", fields=[field("job", "42")])
+let plain = rec.with_fields([])
+```
+
+In this example, the returned record keeps the same event identity but carries no fields.
+
+### Error Case
+
+e.g.:
+- If `fields` is empty, the returned record is still valid and simply carries no structured metadata.
+
+- If code should append or merge fields instead of replacing them, `append_fields(...)` or a custom patch is the better fit.
+
+### Notes
+
+1. Use this helper when one record should swap its entire field array.
+
+2. Use `append_fields(...)` when enrichment should preserve the original field list and add more entries afterward.