- The change was added in #23099

Changed options for find_each and variants to have options start/finish instead of start_at/end_at based on comments
at https://github.com/rails/rails/pull/12257#issuecomment-74688344

`in_batches` yields Relation objects if a block is given, otherwise it
returns an instance of `BatchEnumerator`. The existing `find_each` and
`find_in_batches` methods work with batches of records. The new API
allows working with relation batches as well.

Examples:

    Person.in_batches.each_record(&:party_all_night!)
    Person.in_batches.update_all(awesome: true)
    Person.in_batches.delete_all
    Person.in_batches.map do |relation|
      relation.delete_all
      sleep 10 # Throttles the delete queries
    end

the newer method used for discriminating new records did not
use the older and more robust method used for instantiating
existing records, but did have a better post-check to ensure
the sublass was in the hierarchy. so move the descendants check
to find_sti_class, and then simply call find_sti_class from
subclass_from_attributes

now with fixed specs

Deprecated passing of `start` value to `find_in_batches` and `find_each` in favour of `begin_at` value.

that complements the `start`parameter to specify where to stop batch processing

Before this patch find_in_batches raises this error only on second iteration. So you will know about the problem only when you get the batch size threshold.

can be chained with other `Enumerable` methods.

This lets us do things like call: .find_each.with_index

Related to 761bc751 and
eb876c4d

This reverts commit 761bc751.

This commit wasn't fixing any issue just using the same table for
different models with different primary keys.

On reflection, it seems like a bit of a weird method to have on
ActiveRecord::Base, and it shouldn't be needed most of the time anyway.

Previously it returned an Array.

If you want an array, call e.g. `Post.to_a` rather than `Post.all`. This
is more explicit.

In most cases this should not break existing code, since
Relations use method_missing to delegate unknown methods to #to_a
anyway.

Fixes #2832

This caused that `find_each` was producing extra db call taking all the records from db, and was less efficient than `ActiveRecord::Base#all`.

Currently if the code that calls .find_in_batches modifies the yielded array in place then .find_in_batches can enter an infinite loop searching with ruby object ids in the database instead of the primary key of records in the database. This happens because it naively assumes the yielded array hasn't been modified before calling #id on the last object in the array. And ruby (1.8 at least) alias' #id to #object_id so an integer is still returned no matter what the last object is.

By moving finding the #id of the last object before yielding the array it means the calling code can do whatever it wants to the array in terms of modifying it in place, and .find_in_batches doesn't care.

Refactor Active Record test connection setup. Please see the RUNNING_UNIT_TESTS file for details, but essentially you can now configure things in test/config.yml. You can also run tests directly via the command line, e.g. ruby path/to/test.rb (no rake needed, uses default db connection from test/config.yml). This will help us fix the CI by enabling us to isolate the different Rails versions to different databases.

Fix the tests (I have actually verified that these are also the 'right' fixes, rather than just making the tests pass again)

corrected AR find_each and find_in_batches to raise when the user uses select but does not specify the primary key
Signed-off-by: NJosé Valim <jose.valim@gmail.com>