Clarify parent link in Features context

[m-mohr]

As far as I know the parent links in the context of STAC API - Features must always be pointing to valid Catalogs or Collections.
So the parent links must be as follows:

• parent of /collections/:collectionid/items/:itemid must be /collections/:collectionid
• parent of /collections/:collectionid/items must be /collections/:collectionid
• parent of /collections/:collectionid must be /
• parent of /collections must be /

I often see items pointing to .../items and collections to /collections. I think we should clarify this.

[philvarner]

I've added documentation to clarify these two:

• parent of /collections/:collectionid must be /
• parent of /collections/:collectionid/items/:itemid must be /collections/:collectionid

I made it clear in the table that these two do not have parent links:

• parent of /collections/:collectionid/items must be /collections/:collectionid
• parent of /collections must be /

in OAFeat, /collections/:collectionid/items has a link rel collection so that it's not confused the the parent-child relation. We could add something similar to /collections (catalog?), but there's already root which points to the same link.

[tschaub]

The item spec includes this requirement:

An Item must also provide a link to the STAC Collection using the collection relation type

Then in STAC API, the link from an item to its collection has the parent relation type.

I think STAC Browser assumes the link from item to collection will have the collection relation. Not sure what else makes this assumption.

Should this be parent or collection?

[philvarner]

collection is the right relation if you want to definitely find the Collection. The parent link may link to a parent Catalog the Item is in (though in practice, none of the STAC API impls have sub-Catalog support yet, so it's just root Catalog -> Collections -> Items).

[tschaub]

Thanks for the clarification, @philvarner. I've opened #404 proposing that collection be used instead of parent for links from an item to its collection.

[philvarner]

It's not instead of though -- they're frequently the same (and always the same in all the STAC API impls I know of), but they don't have to be the same -- collection will always point to the Collection resource, but parent may not point to the collection, but might point to a parent Catalog.

I think we need to add to the definitions that there should be a collection link rel (as defined by STAC) in addition to parent (defined by STAC API)