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Nanaloveyuki
80 lines
2.0 KiB
Markdown
80 lines
2.0 KiB
Markdown
---
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name: has-field
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group: api
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category: filtering
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update-time: 20260512
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description: Create a reusable record predicate that checks whether a field key exists.
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key-word:
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- field
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- filter
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- predicate
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- public
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---
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## Has-field
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Create a `RecordPredicate` that returns `true` when a record contains at least one field with the given key. Use it when presence alone matters more than a specific field value.
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### Interface
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```moonbit
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pub fn has_field(key : String) -> RecordPredicate {}
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```
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#### input
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- `key : String` - Field key that should exist in the record.
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#### output
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- `RecordPredicate` - Predicate that matches records containing a field whose key equals `key`.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- The predicate scans `rec.fields` in order and stops on the first matching key.
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- Only the key is checked; the field value is ignored.
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- This helper works well for optional metadata such as `request_id`, `tenant`, or `trace_id`.
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- It can be combined with `field_equals(...)` when some records need stricter field matching.
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### How to Use
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Here are some specific examples provided.
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#### When Keep Records Carrying Correlation Data
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When only records with tracing context should pass:
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```moonbit
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let logger = Logger::new(console_sink())
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.with_filter(has_field("trace_id"))
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```
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In this example, records without `trace_id` are filtered out.
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#### When Combine With Severity Rules
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When contextual records should also meet a level threshold:
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```moonbit
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let predicate = all_of([
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has_field("request_id"),
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level_at_least(Level::Warn),
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])
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```
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In this example, only warning-or-higher records with request context remain.
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### Error Case
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e.g.:
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- If `key` is empty, the predicate only matches fields whose key is also empty.
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- If the same key appears multiple times, the predicate still returns `true` after the first match.
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### Notes
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1. Use this helper when field presence is meaningful even if the actual value changes every time.
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2. Field ordering does not change the result, only the early return cost.
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