Insert into tables in batches

Sometimes it is necessary to store large amounts of records at once, which can be inefficient when iterating collections and saving each record individually. With the arrival of insert_all in Rails 6, which operates at the row level (that is, using Hash objects), GitLab has added a set of APIs that make it safe and simple to insert ActiveRecord objects in bulk.

Prepare ApplicationRecords for bulk insertion

In order for a model class to take advantage of the bulk insertion API, it has to include the BulkInsertSafe concern first:

class MyModel < ApplicationRecord
  # other includes here
  # ...
  include BulkInsertSafe # include this last

  # ...

The BulkInsertSafe concern has two functions:

  • It performs checks against your model class to ensure that it does not use ActiveRecord APIs that are not safe to use with respect to bulk insertions (more on that below).
  • It adds new class methods bulk_insert! and bulk_upsert!, which you can use to insert many records at once.

Insert records with bulk_insert! and bulk_upsert!

If the target class passes the checks performed by BulkInsertSafe, you can insert an array of ActiveRecord model objects as follows:

records = [, ...]


Note that calls to bulk_insert! will always attempt to insert new records. If instead you would like to replace existing records with new values, while still inserting those that do not already exist, then you can use bulk_upsert!:

records = [, existing_model, ...]

MyModel.bulk_upsert!(records, unique_by: [:name])

In this example, unique_by specifies the columns by which records are considered to be unique and as such will be updated if they existed prior to insertion. For example, if existing_model has a name attribute, and if a record with the same name value already exists, its fields will be updated with those of existing_model.

The unique_by parameter can also be passed as a Symbol, in which case it specifies a database index by which a column is considered unique:

MyModel.bulk_insert!(records, unique_by: :index_on_name)

Record validation

The bulk_insert! method guarantees that records will be inserted transactionally, and will run validations on each record prior to insertion. If any record fails to validate, an error is raised and the transaction is rolled back. You can turn off validations via the :validate option:

MyModel.bulk_insert!(records, validate: false)

Batch size configuration

In those cases where the number of records is above a given threshold, insertions will occur in multiple batches. The default batch size is defined in BulkInsertSafe::DEFAULT_BATCH_SIZE. Assuming a default threshold of 500, inserting 950 records would result in two batches being written sequentially (of size 500 and 450 respectively.) You can override the default batch size via the :batch_size option:

MyModel.bulk_insert!(records, batch_size: 100)

Assuming the same number of 950 records, this would result in 10 batches being written instead. Since this will also affect the number of INSERTs that occur, make sure you measure the performance impact this might have on your code. There is a trade-off between the number of INSERT statements the database has to process and the size and cost of each INSERT.

Handling duplicate records

Note: This parameter applies only to bulk_insert!. If you intend to update existing records, use bulk_upsert! instead.

It may happen that some records you are trying to insert already exist, which would result in primary key conflicts. There are two ways to address this problem: failing fast by raising an error or skipping duplicate records. The default behavior of bulk_insert! is to fail fast and raise an ActiveRecord::RecordNotUnique error.

If this is undesirable, you can instead skip duplicate records with the skip_duplicates flag:

MyModel.bulk_insert!(records, skip_duplicates: true)

Requirements for safe bulk insertions

Large parts of ActiveRecord’s persistence API are built around the notion of callbacks. Many of these callbacks fire in response to model life cycle events such as save or create. These callbacks cannot be used with bulk insertions, since they are meant to be called for every instance that is saved or created. Since these events do not fire when records are inserted in bulk, we currently disallow their use.

The specifics around which callbacks are explicitly allowed are defined in BulkInsertSafe. Consult the module source code for details. If your class uses callbacks that are not explicitly designated safe and you include BulkInsertSafe the application will fail with an error.

BulkInsertSafe versus InsertAll

Internally, BulkInsertSafe is based on InsertAll, and you may wonder when to choose the former over the latter. To help you make the decision, the key differences between these classes are listed in the table below.

  Input type Validates input Specify batch size Can bypass callbacks Transactional
bulk_insert! ActiveRecord objects Yes (optional) Yes (optional) No (prevents unsafe callback use) Yes
insert_all! Attribute hashes No No Yes Yes

To summarize, BulkInsertSafe moves bulk inserts closer to how ActiveRecord objects and inserts would normally behave. However, if all you need is to insert raw data in bulk, then insert_all is more efficient.

Insert has_many associations in bulk

A common use case is to save collections of associated relations through the owner side of the relation, where the owned relation is associated to the owner through the has_many class method:

owner = array_of_owned_relations)
# saves all `owned_relations` one-by-one!

This will issue a single INSERT, and transaction, for every record in owned_relations, which is inefficient if array_of_owned_relations is large. To remedy this, the BulkInsertableAssociations concern can be used to declare that the owner defines associations that are safe for bulk insertion:

class OwnerModel < ApplicationRecord
  # other includes here
  # ...
  include BulkInsertableAssociations # include this last

  has_many :my_models

Here my_models must be declared BulkInsertSafe (as described previously) for bulk insertions to happen. You can now insert any yet unsaved records as follows:

BulkInsertableAssociations.with_bulk_insert do
  owner = array_of_my_model_instances)
  # saves `my_models` using a single bulk insert (possibly via multiple batches)!

Note that you can still save relations that are not BulkInsertSafe in this block; they will simply be treated as if you had invoked save from outside the block.

Known limitations

There are a few restrictions to how these APIs can be used:

  • BulkInsertableAssociations:
    • It is currently only compatible with has_many relations.
    • It does not yet support has_many through: ... relations.

Moreover, input data should either be limited to around 1000 records at most, or already batched prior to calling bulk insert. The INSERT statement will run in a single transaction, so for large amounts of records it may negatively affect database stability.