Is it advisable to use to_a with lazy loading in the context of distribute_reads?

[ChandanMangoApps]

Hey @ankane, is it advisable to use to_a with lazy loading in the context of distribute_reads?

[Andrew Kane]

Hi @chandanmangoapps, I don't really understand the question. Can you share more context?

[ChandanMangoApps]

@ankane Sure!
When utilizing lazy loading in a system, the essence lies in deferring the execution of certain operations until they are explicitly needed, allowing for more efficient resource utilization. However, if the to_a method is applied, it triggers immediate execution, bypassing the lazy loading mechanism. In other words, by converting a lazy-loaded query to an array (to_a), we forfeit the benefits of lazy loading, as the operation is no longer deferred but executed promptly. This is particularly relevant in scenarios like ActiveRecord queries, where lazy loading can optimize database access by delaying the execution of queries until necessary. Consider the following example:

Lazy loading example

lazy_query = User.where(active: true) # Does not execute immediately

Using to_a triggers immediate execution

result_array = lazy_query.to_a # Executes the query immediately
In this example, the User.where(active: true) query is lazily loaded and not executed until necessary. However, when to_a is applied, the query is promptly executed, disregarding the lazy loading strategy. Therefore, the decision to use to_a should be made carefully, considering the intended benefits of lazy loading in the specific context of the application.
Given that using to_a with lazy loading effectively negates the advantages of deferring query execution until necessary, I am curious about the advisability of intentionally losing the flavour of lazy loading. Could you provide insights into scenarios where sacrificing lazy loading might be justified or situations where maintaining lazy loading remains crucial for optimal performance and resource utilisation?

[Andrew Kane]

Hi @chandanmangoapps, for a query to go to a replica, it needs to execute within a distribute_reads block (unless distributing reads by default) and calling to_a is a way to ensure that. See this section for more info.