BitLogger/docs/api/has-field.md

name, group, category, update-time, description, key-word

name	group	category	update-time	description	key-word
has-field	api	filtering	20260512	Create a reusable record predicate that checks whether a field key exists.	field filter predicate public

Has-field

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.

Interface

pub fn has_field(key : String) -> RecordPredicate {}

input

• key : String - Field key that should exist in the record.

output

• RecordPredicate - Predicate that matches records containing a field whose key equals key.

Explanation

Detailed rules explaining key parameters and behaviors

• The predicate scans rec.fields in order and stops on the first matching key.
• Only the key is checked; the field value is ignored.
• This helper works well for optional metadata such as request_id, tenant, or trace_id.
• It can be combined with field_equals(...) when some records need stricter field matching.

How to Use

Here are some specific examples provided.

When Keep Records Carrying Correlation Data

When only records with tracing context should pass:

let logger = Logger::new(console_sink())
  .with_filter(has_field("trace_id"))

In this example, records without trace_id are filtered out.

When Combine With Severity Rules

When contextual records should also meet a level threshold:

let predicate = all_of([
  has_field("request_id"),
  level_at_least(Level::Warn),
])

In this example, only warning-or-higher records with request context remain.

Error Case

e.g.:

• If key is empty, the predicate only matches fields whose key is also empty.

• If the same key appears multiple times, the predicate still returns true after the first match.

Notes

1. Use this helper when field presence is meaningful even if the actual value changes every time.

2. Field ordering does not change the result, only the early return cost.