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Replace internal implementation of eager_graph with much faster version

In my testing, this is anywhere from 75% to 225% faster than the
previous implementation (which hasn't changed much since its release
in Sequel 1.5.0).  Before, eager_graph relied on graph_each to
split the hash into subhashes and turn each subhash into a model
object.  However, in many cases (unless you are only eager graphing
*_to_one associations), that is wasteful, as many of the instances
don't get used.

This new approach keeps the plain hash all the way through to the
eager_graph stage.  It uses the metadata provided by graph to parse
the hash into subhashes and create model objects, but only does so
if it is needed (i.e. the model object has not been seen before).
That's the primary strategic optimization.  I have also made numerous
micro-optimizations to reduce the number of method calls and hash

Before, the loading was done purely in Dataset methods.  Now, there
is a new class called EagerGraphLoader that handles the specialized

This is backwards compatible except in the case where you call a
method other than all on the eager graphed dataset.  Previously,
you got a hash with table alias keys and model instance values,
like you would for a plain graph call.  Now, you just get a plain
hash.  I apologize to anyone relying on the previous behavior, but
I think the performance improvement merits breaking backwards
compatibility in this case, especially considering how unlikely it
is that people are relying on the behavior.

There's minor fallout from this change in the sharding plugin, and
significant fallout in the identity_map plugin, but this commit
includes fixes for the fallout in both cases.
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1 parent b0f2ed0 commit 99beb68ecef93dd6d721ce74cc3463f20a516c72 @jeremyevans committed Jul 16, 2011
@@ -1,5 +1,7 @@
=== HEAD
+* Replace internal implementation of eager_graph with much faster version (jeremyevans)
* Don't treat strings with leading zeros as octal format in the default typecasting (jeremyevans)
* Fix literalization of Date, Time, and DateTime values on Microsoft Access (jeremyevans)
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