`bug: FilterError when comparing string metadata dates with datetime objects using ordering operators`

[rautaditya2606]

Describe the bug

document_matches_filter raises a FilterError when comparing a string date field in Document.meta against a datetime object using ordering operators (>, >=, <, <=), even though storing dates as ISO strings in metadata is a common and valid pattern.

Error message

FilterError: Can't compare strings using operators '>', '>=', '<', '<='. Strings are only comparable if they are ISO formatted dates.

To Reproduce

from datetime import datetime, timezone
from haystack.dataclasses import Document
from haystack.utils.filters import document_matches_filter

doc = Document(content="test", meta={"date": "2024-01-01"})
filters = {
    "field": "meta.date",
    "operator": ">",
    "value": datetime(2023, 1, 1, tzinfo=timezone.utc)
}

document_matches_filter(filters, doc)  # Raises FilterError

Expected behavior

The filter should parse the string operand into a datetime object and perform the comparison successfully.

Root Cause

In haystack/utils/filters.py, _prepare_ordering_comparison calls _parse_date on both operands unconditionally if either is a string. When the other operand is already a datetime object, _parse_date fails because it expects a string input.

Fix

Conditionally call _parse_date only when the operand is actually a string:

value = _parse_date(value) if isinstance(value, str) else value
filter_value = _parse_date(filter_value) if isinstance(filter_value, str) else filter_value

Additionally normalize timezone awareness when both resolve to datetime to prevent TypeError on naive/aware comparisons.

Additional context

Only affects mixed-type comparisons where one operand is a string ISO date and the other is a datetime object. Pure string-to-string and datetime-to-datetime comparisons are unaffected.

System:

• OS: any
• Haystack version: 2.31.0-rc0
• Component: document_matches_filter, InMemoryDocumentStore, MetadataRouter

[rautaditya2606]

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