Rails and ActiveSupport Notes

[ohler55]

Please place notes about dealing with Rails and ActiveSupport in this discussion topic.

[RamilGN]

What is the main difference between Oj.dump and Oj::Rails.encode?

[HeyNonster]

Oj.dump, using the default settings, will not return the same format that Rails' ActiveSupport will.

ActiveSupport::JSON.encode(Time.now.utc)                      => "2022-01-20T09:32:27.708Z"
Oj.dump(Time.now.utc)                                         => {"^t":1642671147.709}

However, Oj::Rails.encode will mimic the format of ActiveSupport:

ActiveSupport::JSON.encode(Time.now.utc)                      => "2022-01-20T09:32:27.708Z"
Oj::Rails.encode(Time.now.utc)                                => "2022-01-20T09:32:27.708Z"

But if you set Oj with mode: :rails then Oj.dump will also mimic ActiveSupport.

ActiveSupport::JSON.encode(Time.now.utc)                      => "2022-01-20T09:36:15.727Z"
Oj.dump(Time.now.utc)                                         => "2022-01-20T09:36:15.727Z"

Basically, if you don't want to set mode: :rails but still want Oj to return the same format as ActiveSupport then you can use Oj::Rails.encode. I'm pretty sure that's how it works, at least.

[ohler55]

The interaction between Rails and the JSON gem can be somewhat unpredictable. Oj does it's best to reproduce that. As pointed out Oj.dump is different than the ActiveSupport calls and different than the JSON gem although with the options set correctly it can give the same results. The options are the key.

[HeyNonster]

Hey, everyone, this write-up mostly relates to a caveat when using Oj version <= 3.10.0 with Rails/ActiveSupport and Oj.optimize_rails but may also provide insight into what Oj is doing under the hood if that's of interest.

TL/DR: InOj version <= 3.10.0 passing a time object to JSON.dump and passing a time object nested inside an array or hash to JSON.dump will not return the same time format. This is because of all the monkey patching that ActiveSupport does and was fixed in Oj 3.10.1

Extended details below:

My understanding is that with Oj.optimize_rails Oj will replace JSON.dump && ActiveSupport::JSON.encode with faster implementations written in C. Without Oj.optimize_rails Oj will provide fast Oj.dump methods but otherwise leave everything else alone.

Here's a demonstration with these Oj settings: Oj.default_options = { mode: :rails, trace: true } and with ActiveSupport

# Oj version 3.10.0

#JSON.dump
JSON.dump(Time.now.utc)                                => "2022-01-11 12:17:36 UTC"

# ActiveSupport::JSON.encode
ActiveSupport::JSON.encode(Time.now.utc)               => "2022-01-11T12:17:36.614Z"

# Oj.dump
#0:      rails.c:1468:Oj:}: dump Time
#0:      rails.c:517:Oj:>:   as_json Time
#0:      rails.c:527:Oj:<:   as_json Time
#0:      rails.c:1468:Oj:}: dump String
#0:      rails.c:1479:Oj:{: dump String
#0:      rails.c:1479:Oj:{: dump Time
Oj.dump(Time.now.utc)                                  => "2022-01-11T12:17:36.614Z"

Notice above the Oj.dump(Time.now.utc) line. The trace: true Oj option gives you a window into what Oj is doing and you can see here that Oj.dump utilizes the fast C code. Notably, the file is rails.c. This is because we set mode: :rails in the default_options which tells Oj to render the JSON in the same format that ActiveSupport would.

Now, if we run the same code but add Oj.optimize_rails:

# Oj version 3.10.0

#JSON.dump
#0:dump_compat.c:940:Oj:}: dump Time
#0:dump_compat.c:965:Oj:{: dump Time
JSON.dump(Time.now.utc)                                => "2022-01-11 12:27:17 UTC"

# ActiveSupport::JSON.encode
#0:      rails.c:1468:Oj:}: dump Time
#0:      rails.c:1479:Oj:{: dump Time
ActiveSupport::JSON.encode(Time.now.utc)               => "2022-01-11T12:27:17.051Z"

# Oj.dump
#0:      rails.c:1468:Oj:}: dump Time
#0:      rails.c:1479:Oj:{: dump Time
Oj.dump(Time.now.utc)                                  => "2022-01-11T12:27:17.051Z"

Both JSON.dump and ActiveSupport::JSON.encode now run through faster C implementations. JSON.dump,
however runs through the file dump_compat.c instead of rails.c. This is intended so that JSON.dump with Oj and JSON.dump without Oj return the same time format, the Oj implementation is just faster.

The bug in Oj < 3.10.1 occured when you passed a Hash or Array with time objects to JSON.dump

# Oj version 3.10.0

# JSON.dump
#0:dump_compat.c:940:Oj:}: dump Hash
#0:dump_compat.c:940:Oj:}:   dump Time
#0:dump_compat.c:123:Oj:>: to_json Time
#0:      rails.c:1468:Oj:}: dump Time
#0:      rails.c:1479:Oj:{: dump Time
#0:dump_compat.c:131:Oj:<: to_json Time
#0:dump_compat.c:965:Oj:{:   dump Time
#0:dump_compat.c:965:Oj:{: dump Hash
JSON.dump({created_at: Time.now.utc})                  => {"created_at":"2022-01-11T12:27:16.966Z"}

As you can see in the trace, JSON.dump({created_at: Time.now.utc}) was running the code through both dump_compat.c and rails.c and returning the Time in the ActiveSupport format when it was supposed to return the same format that the JSON standard library gem would.

This was fixed in Oj 3.10.1 as you can see in the below trace:

# Oj version 3.10.1

# JSON.dump
#0:dump_compat.c:941:Oj:}: dump Hash
#0:dump_compat.c:941:Oj:}:   dump Time
#0:dump_compat.c:123:Oj:>: to_json Time
#0:dump_compat.c:131:Oj:<: to_json Time
#0:dump_compat.c:966:Oj:{:   dump Time
#0:dump_compat.c:966:Oj:{: dump Hash
JSON.dump({created_at: Time.now.utc})                  => {"created_at":"2022-01-11 12:34:36 UTC"}

JSON.dump({created_at: Time.now.utc}) now exclusively uses the dump_compat.c file which was the intended behavior. Unfortunately, we had some code that relied on the time format produced by the buggy version of JSON.dump and that broke some things when we upgraded Oj.

Because Oj has to mimic the JSON standard library gem time format and, separately, mimic the ActiveSupport time format (both of which Oj has no control over, it just needs to adapt) we might write a test that catches whenever one the formats change.