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**Executive Summary.** When the elements in a _section_ are combined
into a _chunk_, the metadata in each of the elements is _consolidated_
into a single `ElementMetadata` instance. There are two main problems
with the current implementation:
1. The current algorithm simply uses the metadata of the first element
as the metadata for the chunk. This produces:
- **empty chunk metadata** when the first element has no metadata, such
as a `PageBreak("")`
- **missing chunk metadata** when the first element contains only
partial metadata such as a `Header()` or `Footer()`
- **misleading metadata** when the first element contains values
applicable only to that element, such as `category_depth`, `coordinates`
(bounding-box), `header_footer_type`, or `parent_id`
2. Second, list metadata such as `emphasized_text_content`,
`emphasized_text_tags`, `link_texts` and `link_urls` is only combined
when it is unique within the combined list. These lists are "unzipped"
pairs. For example, the first `link_texts` corresponds to the first
`link_urls` value. When an item is removed from one (because it matches
a prior entry) and not the other (say same text "here" but different
URL) the positional correspondence is broken and downstream processing
will at best be wrong, at worst raise an exception.
### Technical Discussion
Element metadata cannot be determined in the general case simply by
sampling that of the first element. At the same time, a simple union of
all values is also not sufficient. To effectively consolidate the
current variety of metadata fields we need four distinct strategies,
selecting which to apply to each field based on that fields provenance
and other characteristics.
The four strategies are:
- `FIRST` - Select the first non-`None` value across all the elements.
Several fields are determined by the document source (`filename`,
`file_directory`, etc.) and will not change within the output of a
single partitioning run. They might not appear in every element, but
they will be the same whenever they do appear. This strategy takes the
first one that appears, if any, as proxy for the value for the entire
chunk.
- `LIST` - Consolidate the four list fields like
`emphasized_text_content` and `link_urls` by concatenating them in
element order (no set semantics apply). All values from `elements[n]`
appear before those from `elements[n+1]` and existing order is
preserved.
- `LIST_UNIQUE` - Combine only unique elements across the (list) values
of the elements, preserving order in which a unique item first appeared.
- `REGEX` - Regex metadata has its own rules, including adjusting the
`start` and `end` offset of each match based its new position in the
concatenated text.
- `DROP` - Not all metadata can or should appear in a chunk. For
example, a chunk cannot be guaranteed to have a single `category_depth`
or `parent_id`.
Other strategies such as `COORDINATES` could be added to consolidate the
bounding box of the chunk from the coordinates of its elements, roughly
`min(lefts)`, `max(rights)`, etc. Others could be `LAST`, `MAJORITY`, or
`SUM` depending on how metadata evolves.
The proposed strategy assignments are these:
- `attached_to_filename`: FIRST,
- `category_depth`: DROP,
- `coordinates`: DROP,
- `data_source`: FIRST,
- `detection_class_prob`: DROP, # -- ? confirm --
- `detection_origin`: DROP, # -- ? confirm --
- `emphasized_text_contents`: LIST,
- `emphasized_text_tags`: LIST,
- `file_directory`: FIRST,
- `filename`: FIRST,
- `filetype`: FIRST,
- `header_footer_type`: DROP,
- `image_path`: DROP,
- `is_continuation`: DROP, # -- not expected, added by chunking, not
before --
- `languages`: LIST_UNIQUE,
- `last_modified`: FIRST,
- `link_texts`: LIST,
- `link_urls`: LIST,
- `links`: DROP, # -- deprecated field --
- `max_characters`: DROP, # -- unused in code, probably remove from
ElementMetadata --
- `page_name`: FIRST,
- `page_number`: FIRST,
- `parent_id`: DROP,
- `regex_metadata`: REGEX,
- `section`: FIRST, # -- section unconditionally breaks on new section
--
- `sent_from`: FIRST,
- `sent_to`: FIRST,
- `subject`: FIRST,
- `text_as_html`: DROP, # -- not expected, only occurs in TableSection
--
- `url`: FIRST,
**Assumptions:**
- each .eml file is partitioned->chunked separately (not in batches),
therefore
sent-from, sent-to, and subject will not change within a section.
### Implementation
Implementation of this behavior requires two steps:
1. **Collect** all non-`None` values from all elements, each in a
sequence by field-name. Fields not populated in any of the elements do
not appear in the collection.
```python
all_meta = {
"filename": ["memo.docx", "memo.docx"]
"link_texts": [["here", "here"], ["and here"]]
"parent_id": ["f273a7cb", "808b4ced"]
}
```
2. **Apply** the specified strategy to each item in the overall
collection to produce the consolidated chunk meta (see implementation).
### Factoring
For the following reasons, the implementation of metadata consolidation
is extracted from its current location in `chunk_by_title()` to a
handful of collaborating methods in `_TextSection`.
- The current implementation of metadata consolidation "inline" in
`chunk_by_title()` already has too many moving pieces to be understood
without extended study. Adding strategies to that would make it worse.
- `_TextSection` is the only section type where metadata is consolidated
(the other two types always have exactly one element so already exactly
one metadata.)
- `_TextSection` is already the expert on all the information required
to consolidate metadata, in particular the elements that make up the
section and their text.
Some other problems were also fixed in that transition, such as mutation
of elements during the consolidation process.
### Technical Risk: adding new `ElementMetadata` field breaks metadata
If each metadata field requires a strategy assignment to be consolidated
and a developer adds a new `ElementMetadata` field without adding a
corresponding strategy mapping, metadata consolidation could break or
produce incorrect results.
This risk can be mitigated multiple ways:
1. Add a test that verifies a strategy is defined for each
(Recommended).
2. Define a default strategy, either `DROP` or `FIRST` for scalar types,
`LIST` for list types.
3. Raise an exception when an unknown metadata field is encountered.
This PR implements option 1 such that a developer will be notified
before merge if they add a new metadata field but do not define a
strategy for it.
### Other Considerations
- If end-users can in-future add arbitrary metadata fields _before_
chunking, then we'll need to define metadata-consolidation behavior for
such fields. Depending on how we implement user-defined metadata fields
we might:
- Require explicit definition of a new metadata field before use,
perhaps with a method like `ElementMetadata.add_custom_field()` which
requires a consolidation strategy to be defined (and/or has a default
value).
- Have a default strategy, perhaps `DROP` or `FIRST`, or `LIST` if the
field is type `list`.
### Further Context
Metadata is only consolidated for `TextSection` because the other two
section types (`TableSection` and `NonTextSection`) can only contain a
single element.
---
## Further discussion on consolidation strategy by field
### document-static
These fields are very likely to be the same for all elements in a single
document:
- `attached_to_filename`
- `data_source`
- `file_directory`
- `filename`
- `filetype`
- `last_modified`
- `sent_from`
- `sent_to`
- `subject`
- `url`
*Consolidation strategy:* `FIRST` - use first one found, if any.
### section-static
These fields are very likely to be the same for all elements in a single
section, which is the scope we really care about for metadata
consolidation:
- `section` - an EPUB document-section unconditionally starts new
section.
*Consolidation strategy:* `FIRST` - use first one found, if any.
### consolidated list-items
These `List` fields are consolidated by concatenating the lists from
each element that has one:
- `emphasized_text_contents`
- `emphasized_text_tags`
- `link_texts`
- `link_urls`
- `regex_metadata` - special case, this one gets indexes adjusted too.
*Consolidation strategy:* `LIST` - concatenate lists across elements.
### dynamic
These fields are likely to hold unique data for each element:
- `category_depth`
- `coordinates`
- `image_path`
- `parent_id`
*Consolidation strategy:*
- `DROP` as likely misleading.
- `COORDINATES` strategy could be added to compute the bounding box from
all bounding boxes.
- Consider allowing if they are all the same, perhaps an `ALL` strategy.
### slow-changing
These fields are somewhere in-between, likely to be common between
multiple elements but varied within a document:
- `header_footer_type` - *strategy:* drop as not-consolidatable
- `languages` - *strategy:* take first occurence
- `page_name` - *strategy:* take first occurence
- `page_number` - *strategy:* take first occurence, will all be the same
when `multipage_sections` is `False`. Worst-case semantics are "this
chunk began on this page".
### N/A
These field types do not figure in metadata-consolidation:
- `detection_class_prob` - I'm thinking this is for debug and should not
appear in chunks, but need confirmation.
- `detection_origin` - for debug only
- `is_continuation` - is _produced_ by chunking, never by partitioning
(not in our code anyway).
- `links` (deprecated, probably should be dropped)
- `max_characters` - is unused as far as I can tell, is unreferenced in
source code. Should be removed from `ElementMetadata` as far as I can
tell.
- `text_as_html` - only appears in a `Table` element, each of which
appears in its own section so needs no consolidation. Never appears in
`TextSection`.
*Consolidation strategy:* `DROP` any that appear (several never will)