This weekend I crossed a Twitter thread that ended up motivating me to write this blog post.

Thanks, find_by works perfectly, but IIRC, ActiveRecord needs an ID field to save/modify data. I’ll try out composite_primary_keys with an ID+Timestamp combo, then. I’ve got a use case where I want to change a record in a hypertable to add metadata info.

In this scenario, there’s an enriching process to add metadata. Generally, time-series data comes from sensors and other sources representing a state of something or action of someone at a point in time. These states and actions are generally unchangeable, so I never overthought them.

On the core concepts of the TimescaleDB, hypertables use local indices instead of having a unique index for the entire hypertable. Check what the official documentation says:

Rather than building a global index over an entire hypertable, TimescaleDB builds local indexes on each chunk. In other words, each chunk has its own index that only indexes data within that chunk. Even with multiple local indexes, TimescaleDB can still ensure global uniqueness for keys. It enforces an important constraint: any key that requires uniqueness, such as a PRIMARY KEY, must include all columns that are used for data partitioning.

So, if you need to update the data, record by record, here is a small idea that might help you make it using update_all.

I will get a random scenario from the test database I used to build the gem.

I have the following ActiveRecord model as an example:

Tick # => Timescaledb::Tick(symbol: text, price: decimal, time: datetime)
tick = Tick.create(symbol: "TEST", price: 123.4, time:
# => #<Timescaledb::Tick:0x00007f9fd96f5358 symbol: "TEST", price: 0.1234e3, time: 2022-10-18 12:56:13.409779 UTC>

Now, if I try to update an attribute of the record:

tick.symbol = "OTHER" # => "OTHER"

And then, when I try to save it:
ActiveRecord::StatementInvalid: PG::SyntaxError: ERROR:  zero-length delimited identifier at or near """"
LINE 1: UPDATE "ticks" SET "symbol" = $1 WHERE "ticks"."" IS NULL
Caused by PG::SyntaxError: ERROR:  zero-length delimited identifier at or near """"
LINE 1: UPDATE "ticks" SET "symbol" = $1 WHERE "ticks"."" IS NULL

Boom! 💥

Here comes the problem. A primary key will be necessary to update it. As the ActiveRecord depends on id to update records, now you need to find a new way to specify it.

For that reason, there’s a gem composite_primary_key that allows you to specify the attributes that should be used in the primary key.

I initially added it to the timescaledb gem, but as most of the use cases will not depend on updates, I dropped the dependency and covered a possible solution using the update_all as a solution.

Understanding the changes

Let’s continue using the Tick model as an example:

class Tick < ActiveRecord::Base
  self.primary_key = nil

  acts_as_hypertable ...

The primary key is nil, but we can get a simple record by limiting it. Probably you’ll also be managing a few records that are already in the memory, so it will work fine:

t = Tick.first # => #<Timescaledb::Tick:0x00007fb0a77a9360 symbol: "TEST", price: 0.1234e3, time: 2022-10-18 12:56:13.409779 UTC>

Now, let’s change the record attributes:

t.symbol = "Other" # => "Other"

Now, you can see the changes are being tracked on memory:

t.changes # => {"symbol"=>["TEST", "Other"]}

If you make more changes, they’ll continue being tracked:

t.price = 999 # => 999
t.changes # => {"symbol"=>["TEST", "Other"], "price"=>[0.1234e3, 0.999e3]}

Now, let’s rebuild hashes to use before and after the changes:

before = t.changes.transform_values(&:first)
# => {"symbol"=>"TEST", "price"=>0.1234e3}
after = t.changes.transform_values(&:last)
# => {"symbol"=>"Other", "price"=>0.999e3}

We still need to combine and merge the values with the attributes:

t.attributes # => {"symbol"=>"Other", "price"=>0.999e3, "time"=>2022-10-18 12:56:13.409779 UTC}

Now, we can build a variable merging the actual attributes which the changes before, meaning where they came from.

from = t.attributes.merge(before)
# => {"symbol"=>"TEST", "price"=>0.1234e3, "time"=>2022-10-18 12:56:13.409779 UTC}

And another variable merging attributes with where they’re going to:

to = t.attributes.merge(after)
# => {"symbol"=>"Other", "price"=>0.999e3, "time"=>2022-10-18 12:56:13.409779 UTC}

Indeed this is the same as attributes, and the to variable could be simply t.attributes, but for learning purposes, let’s use the explicit mode here.

Now, update_all can use select data-building conditions from the original scenario and update all to the new state:

Tick.where(from).update_all(to) # => 1

Note that it can be expensive if you don’t build the proper indices to find the records and you have a huge dataset. Keep in mind this example is for learning purposes, and you’ll need to update and adjust it to your needs if you want to use it in production.

Also, creating the indices will depend on how often you make updates and how fast you need to have the results up to date.

Wrapping update_all into a new save method

Now, to wrap up here, let’s implement the new save method that allows you to reuse the scenario in several models.

class Tick < ActiveRecord::Base
  self.primary_key = nil

  acts_as_hypertable ...

  def save
    if self.class.primary_key ||
       self.class.respond_to?(:primary_keys) && self.class.primary_keys.any?

      return super

    before = changes.transform_values(&:first)
    from = attributes.merge(before)
    to = changes.transform_values(&:last)


Note that in the first lines of the save method, there’s a clause to use the default behavior in case the model is configured with a primary key or has a composite primary key.

The update_all doesn’t run the model validations, so be careful with this kind of usage that is highly dependent on your control. We could make the behavior precisely the same by adding an extra guard clause to return unless valid? at the top of the methods and ensuring it will update only validated records.

The only point I haven’t covered is the clear_changes_information that cleans the changes attribute to track new changes in case the user makes several changes in the same record without reloading it.

If you have any concerns or comments, feel free to drop me a comment on Twitter or reach out on linkedin!

Thanks for reading.

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Hello there, my name is Jônatas Davi Paganini and this is my personal blog.
I'm developer advocate at Timescale and I also have a few open source projects on github.

Check my talks or connect with me via linkedin / twitter / github / instagram / facebook / strava / meetup.