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Ruby on Rails Module 11

Active Record in depth interview questions

Interview questions on active Record in depth: short warm-ups, then what each harder question tests, the first step of a good answer and its follow-ups.

4 questions: 1 warm-up and 3 with model answers · Levels: Fresher to staff

Warm-up questions

Short questions with short answers, each answered in full.

  1. Fresher

    How do you spot an N+1 query while a page is open in development?

    Open the terminal running the server and read the SELECTs the request logged. An N+1 shows up as a cluster of identical statements with different bound values, one per row of the list on the page: SELECT "teachers".* FROM "teachers" WHERE "teachers"."id" = ? LIMIT ? nine times in a row, each with a different ?, right after the one SELECT that read the batches. With config.active_record.query_log_tags_enabled on (a new app sets it in development), each SELECT carries the controller and action that sent it, which turns a cluster of reads into one line you can point at (same tag, repeated). In a test, wrap the request in ActiveSupport::Notifications.subscribed(..., "sql.active_record") and count: a page that was 2 queries this morning and is 11 queries now has a new lazy read a reader should name.

    From the lesson N+1 queries, eager loading and strict loading

Questions with model answers

What each question tests, the first step of a good answer and the follow-ups to expect.

  1. Fresher to mid

    A teammate's pull request keeps swapping includes(:teacher) for preload(:teacher) "to be explicit about the two SELECTs." Walk through when the swap is a no-op, when it is wrong, and what the SQL shape would be in each case.

    What it tests
    Reading the chain to decide between preload and eager_load, knowing when includes picks each one, and naming the exact behaviour of .references on a SQL fragment.
    First step
    Compare the three methods by what SQL they send. includes picks between preload (two SELECTs, no join) and eager_load (one SELECT with a LEFT OUTER JOIN) based on whether anything in the chain references the association's columns. preload always sends two SELECTs; eager_load always sends one SELECT with a join. From there walk through what each one does on the chain in question.
    Follow-ups

    From the lesson N+1 queries, eager loading and strict loading

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    In the answer:

    • Model answer
    • Common mistakes
    • Rubric
    • Variants
    • about 4 minutes

  2. Mid to senior

    An app with 400 pages wants N+1 regressions to stop at the model. How would you roll out strict_loading without surfacing errors to users, and when would you reach for :n_plus_one_only instead of the strict default?

    What it tests
    Choosing between the four places strict_loading lives (relation, record, association, environment), moving an app to it safely with action_on_strict_loading_violation, and knowing which lazy loads :n_plus_one_only still allows.
    First step
    Set config.active_record.strict_loading_by_default = true with action_on_strict_loading_violation = :log. The pair raises nothing on a user's page while every lazy load writes a line in the log the team can read. Fix the pages one by one, each one with an includes, a preload or an eager_load that matches the chain. Flip the setting to the default (:raise) only when the log is clean for a release.
    Follow-ups

    From the lesson N+1 queries, eager loading and strict loading

    In the answer:

    • Model answer
    • Common mistakes
    • Rubric
    • Variants
    • about 4 minutes

  3. Senior to staff

    A teacher list reads teacher.batches.size thousands of times a minute, and a nightly import writes 40,000 rows with insert_all. Decide where size's numbers should come from: a counter cache, a cached SELECT, a materialised view, or a preloaded association, and say what each one costs when the import runs.

    What it tests
    Trading reads against writes with real numbers: how size picks length / count / counter_cache, which writers break which option, and when the fix belongs on the query instead of on the column.
    First step
    List every writer that touches batches, in order: user saves (go through callbacks), the import (insert_all skips callbacks), direct update_all (skips callbacks), and failed transactions (roll back). Then say what each option costs on each writer: a counter cache drifts when callbacks are skipped; a cached SELECT goes stale until expiry; a materialised view needs a refresh job; a preloaded association reads every row.
    Follow-ups

    From the lesson N+1 queries, eager loading and strict loading

    In the answer:

    • Model answer
    • Common mistakes
    • Rubric
    • Variants
    • about 5 minutes

All Ruby on Rails interview questions · The Ruby on Rails track