[Content]: Eager Loading should mention what field types and properties benefit/support it

[AugustMiller]

Product

Other/Multiple/Not Sure

Description

We aren't currently explicitly listing the fields and/or properties of elements that can be eager-loaded.

This may have to be "generic," but should still be front-and-center, i.e. "Only properties and custom fields that reference other elements can be eager-loaded."

Additionally, not all fields and/or related elements need to be eager-loaded. We can cover situations where you are apt to see performance improvements vs situations where you won't see any benefit.

This may include a mention of the eager-loading-in-place service method:

{% do craft.app.elements.eagerLoadElements(
  className(entry),
  [entry],
  [
     ...
  ]
) %}

[MattWilcox]

May also be worth mentioning that if you do try to eager load a field that doesn't support it, you'll get blank data for the field instead of the actual data.

Ask me how I know.

{% do craft.app.elements.eagerLoadElements(
	className(product),
	[product],
	[
		'listingImage',
		'whoopsNotEagerLoadable'
	]
) %}

{% for thing in product.whoopsNotEagerLoadable %}
	...
{% endfor %}

Results in nothing, but if you remove whoopsNotEagerLoadable from the Eager Loading you get the data in that field.