Refactor `Version` Model

[Mr0grog]

The Version model has had a tortured history, starting as a local copy of Versionista’s data, and slowly evolving into something much different. Today it could probably use some cleanup.

Normally I’d say this is not worth the bugs it might introduce, but we developer involvement here is waning, and I think it’s valuable to do the work to make the data easier to learn and understand, whether that’s to make future development by other programmers easier to get started with or to make dead, archived data easier to use.

Current fields:

✅ = all good as-is, ⚠️ = should change, :x: = should remove, 🌱 = should add

Status	Field	Category	Usage/Notes
✅	uuid	DB-centric	Acts as a convenient, single-field primary key, even though versions can also be uniquely identified by (capture_time, capture_url, source_type)
✅	created_at	DB-centric	Should never be modified; indicates when the record was created.
✅	updated_at	DB-centric	Indicates when the record was last modified. Only relevant insofar as we have non-canonical fields listed below.
✅	capture_time	Canonical	The time at which the version was originally captured in the source system. Component of the natural key.
⚠️	capture_url	Canonical	The URL this is a capture of, e.g. https://epa.gov/climate-change/. I think we should rename this to url, which would be more concise and clearer. Component of the natural key.
✅	source_type	Canonical	The system that originally captured this, e.g. internet_archive. Component of the natural key.
✅	status	Canonical	The status code of the HTTP response.
⚠️	uri	Canonical	The location of the captured HTTP response body. I think we should rename this to body_url for clarity. Everyone new gets tripped up on this one.
⚠️	version_hash	Canonical	Technically this is derived data, but it should be treated as canonical since we use it for fixity. I think we should rename this to body_hash for clarity, and to go with the suggested rename of uri to body_url above.
✅	source_metadata	Canonical	Arbitrary additional metadata from the source that we don’t have a good field for on Version.
✅	page_uuid	Derived	Note this is not canonical! Pages are really an arbitrary construct. We also want to start treating this as something user settable (rather than something exclusively calculated from underlying data), because pages having multiple URLs (#492) means pages are mergeable, and the page a version belongs to is a malleable concept.
✅	title	Derived	Title of the page/document/whatever, if we can figure it out.
⚠️	content_length	Derived	The actual size in bytes of the HTTP response body (not the Content-Length header). Doesn’t reside anywhere else in the DB, but is/can be automatically derived from the data found from the uri field. I’d like to suggest renaming this to body_length or body_size for consistency with the other body_* fields suggested above and to differentiate from the header of the same name, but don’t think it’s a big deal. The current name is not a problem.
✅	media_type	Derived	Currently the parsed media type from the Content-Type header. I don’t think the name should change, although we should consider sniffing content and canonicalizing the type here instead of just reflecting the header. (See also #752)
:x:	media_type_parameters	Denormalized	This is complicated to use, not typically of much value, and not worth surfacing directly on the model. If someone ever has an actual use for it, they should get it by consulting the headers. We should remove it. (See also #752)
✅	different	Denormalized	This is the only field that merely denormalizes data found elsewhere in the DB, but we currently use it in most of our queries.
🌱	headers	Canonical	Does not exist/should add. The headers of the HTTP response. Having this goes a long way towards clarifying Version as a canonical record of an archived HTTP response, which is what it should really be at this point. We do not always have headers (so this must be nullable, or maybe an empty object?), but when we do they should be here instead of in source_metadata.
~🌱~	~charset~	Derived	~Does not exist/should add. Character encoding of the HTTP response body. I’m less bullish on this one, since we can set it in the response headers of uri/body_url and, for HTML, it is often specified (and specified more correctly) in the document itself. It can also be retrieved from the headers in cases where we didn’t have to sniff it during import.~

We also track redirects in source_metadata (so we know if the response is a direct response to capture_url or a response to a redirect from it), but I’m not sure that’s worth pulling up into the main model.

I’ve made a point of calling out DB-centric vs. derived vs. denormalized because I think these are particularly useful distinctions. If there weren’t practical concerns, I’d almost suggest the derived & denormalized data belongs in a second table that has a 1:1 relationship with versions. That way we are separating out the canonical record of an archived HTTP response from other useful data we want to make easily available about it.

If it weren’t a total mess to do, I’d also suggest renaming Version to Capture or Snapshot, but I don’t think that’s worth the complex mess and migration headaches. Renaming fields is small potatoes in comparison.

[Mr0grog]

Actually, the more I think about it, the less I want to propose adding charset.

[Mr0grog]

In a discussion yesterday, @danielballan concurred with the ideas here, so I feel a little better about actually doing it.

The removal of media_type_parameters is already under way with #778.

[Mr0grog]

Things that still need doing here:

• [ ] capture_url → url
• [ ] uri → body_url
• [ ] version_hash → body_hash
• [ ] content_length → body_length
• [ ] Add headers (a.k.a. source_metadata.headers → headers)