Prevent re-adding same transformers to the view all the time

[Pritilender]

While updating to the latest version, I've noticed that performance for rendering a blueprint with a transformer has worsened significantly. Digging around the code, I found that the issue is the usage of concat method which keeps adding and adding same transformers to the view on each call fetching defined transformers. This change is using simple array deconstructing to ensure immutability of the transformers array.

I've also added a case with a transformer to the basic benchmark spec because I think it's valuable to also have such a case there. Let me know if I need to make it more detailed or to also add it to other benchmark specs.

Checklist:

• I have updated the necessary documentation
• I have signed off all my commits as required by DCO
• My build is green

[lessthanjacob]

Thanks for opening a PR to address this! The change seems reasonable to me, but in the meantime, could you add some before-and-after benchmarks to the PR?

[Pritilender]

The benchmark with current code from master fails after a few minutes of running:

➜ bundle exec rake benchmarks
-- create_table(:users, {:force=>:cascade})
   -> 0.0046s
-- create_table(:vehicles, {:force=>:cascade})
   -> 0.0002s
Run options: --seed 37132

# Running:


ActiveRecord IPS: 28183
.bench_render_active_record      0.000065        0.000093        0.000519        0.005195        0.050334
.
Basic IPS: 3
F

Failure:
Blueprinter::IPSTest#test_render [spec/benchmarks/ips_test.rb:32]:
Expected 3 to be >= 2500.


bin/rails test spec/benchmarks/ips_test.rb:29

bench_render_basic       0.000095        0.000128        0.000595        0.004502        0.043274
.

Finished in 163.240825s, 0.0245 runs/s, 0.0245 assertions/s.
4 runs, 4 assertions, 1 failures, 0 errors, 0 skips

After applying the changes, the benchmark passes again with the usual number for IPS on the basic benchmark:

➜ bundle exec rake benchmarks
-- create_table(:users, {:force=>:cascade})
   -> 0.0079s
-- create_table(:vehicles, {:force=>:cascade})
   -> 0.0002s
Run options: --seed 17693

# Running:

bench_render_active_record       0.000101        0.000106        0.000556        0.005278        0.053700
.
ActiveRecord IPS: 26212
.bench_render_basic      0.000049        0.000070        0.000376        0.003469        0.036867
.
Basic IPS: 27383
.

Finished in 4.610934s, 0.8675 runs/s, 0.8675 assertions/s.
4 runs, 4 assertions, 0 failures, 0 errors, 0 skips