Sequel PostgreSQL Triggers¶ ↑

Sequel PostgreSQL Triggers is a small enhancement to Sequel allowing a user to easily handle the following types of columns:

Timestamp Columns (Created At/Updated At)
Counter/Sum Caches
Immutable Columns
Touch Propogation

It handles these internally to the database via triggers, so even if other applications access the database (without using Sequel), things will still work (unless the database superuser disables triggers).

To use this, load the pg_triggers extension into the Sequel::Database object:

DB.extension :pg_triggers

Then you can call the pgt_* methods it adds on your Sequel::Database object:

DB.pgt_created_at(:table_name, :created_at)

Most commonly, this is used in migrations, with a structure similar to:

Sequel.migration do
  up do
    extension :pg_triggers

    pgt_created_at(:table_name,
                   :created_at,
                   :function_name=>:table_name_set_created_at,
                   :trigger_name=>:set_created_at)
  end

  down do
    drop_trigger(:table_name, :set_created_at)
    drop_function(:table_name_set_created_at)
  end
end

Note that you only need to load this extension when defining the triggers, you don’t need to load this extension when your application is running.

To use any of these methods before PostgreSQL 9.0, you have to add the plpgsql procedural language to PostgreSQL, which you can do with:

DB.create_language(:plpgsql)

If you want to load this extension globally for all PostgreSQL databases, you can do:

require 'sequel_postgresql_triggers'

However, global modification is discouraged and only remains for backwards compatibility.

Triggers¶ ↑

All of the public methods this extension adds take the following options in their opts hash:

:function_name: The name of the function to use. This is important to specify if you want an easy way to drop the function.
:trigger_name: The name of the trigger to use. This is important to specify if you want an easy way to drop the trigger.

Created At Columns - pgt_created_at¶ ↑

pgt_created_at takes the table and column given and makes it so that upon insertion, the column is set to the CURRENT_TIMESTAMP, and that upon update, the column’s value is always set to the previous value. This is sort of like an immutable column, but it doesn’t bring up an error if you try to change it, it just ignores it.

Arguments:

table: name of table
column: column in table that should be a created at timestamp column
opts: option hash

Updated At Columns - pgt_updated_at¶ ↑

Similar to pgt_created_at, takes a table and column and makes it so that upon insertion, the column is set to CURRENT_TIMESTAMP. It differs that upon update, the column is also set to CURRENT_TIMESTAMP.

Arguments:

table: name of table
column: column in table that should be a updated at timestamp column
opts: options hash

Counter Cache - pgt_counter_cache¶ ↑

This takes quite a few arguments (see the RDoc) and sets up a counter cache so that when the counted table is inserted to or deleted from, records in the main table are updated with the count of the corresponding records in the counted table. The counter cache column must have a default of 0 for this to work correctly.

Use pgt_sum_cache with a Sequel expression in summed_column to handle any custom logic such as a counter cache that only counts certain rows.

Arguments:

main_table: name of table holding counter cache column
main_table_id_column: column in main table matching counted_table_id_column in counted_table
counter_column: column in main table containing the counter cache
counted_table: name of table being counted
counted_table_id_column: column in counted_table matching main_table_id_column in main_table
opts: options hash

Sum Cache - pgt_sum_cache¶ ↑

Similar to pgt_counter_cache, except instead of storing a count of records in the main table, it stores the sum on one of the columns in summed table. The sum cache column must have a default of 0 for this to work correctly.

Use a Sequel expression in summed_column to handle any custom logic such as a counter cache that only counts certain rows, or a sum cache that sums the length of a string column.

Arguments:

main_table: name of table holding counter cache column
main_table_id_column: column in main table matching counted_table_id_column in counted_table
sum_column: column in main table containing the sum cache
summed_table: name of table being summed
summed_table_id_column: column in summed_table matching main_table_id_column in main_table
summed_column: column in summed_table being summed or a Sequel expression to be evaluated in the context of summed_table
opts: options hash

Sum Through Many Cache - pgt_sum_through_many_cache¶ ↑

Similar to pgt_sum_cache, except instead of a one-to-many relationship, it supports a many-to-many relationship with a single join table. The sum cache column must have a default of 0 for this to work correctly. Use a Sequel expression in summed_column to handle any custom logic. See pgt_sum_cache for details.

This takes a single options hash argument, supporting the following options in addition to the standard options:

:main_table: name of table holding sum cache column
:main_table_id_column: primary key column in main table referenced by main_table_fk_column (default: :id)
:sum_column: column in main table containing the sum cache, must be NOT NULL and default to 0
:summed_table: name of table being summed
:summed_table_id_column: primary key column in summed_table referenced by summed_table_fk_column (default: :id)
:summed_column: column in summed_table being summed or a Sequel expression to be evaluated in the context of summed_table, must be NOT NULL
:join_table: name of table which joins main_table with summed_table
:join_trigger_name: name of trigger for join table
:join_function_name: name of trigger function for join table
:main_table_fk_column: column in join_table referencing main_table_id_column, must be NOT NULL
:summed_table_fk_column: column in join_table referencing summed_table_id_column, must be NOT NULL

Immutable Columns - pgt_immutable¶ ↑

This takes a table name and one or more column names, and adds an update trigger that raises an exception if you try to modify the value of any of the columns.

Arguments:

table: name of table
*columns: All columns in the table that should be immutable. Can end with options hash.

Touch Propagation - pgt_touch¶ ↑

This takes several arguments (again, see the RDoc) and sets up a trigger that watches one table for changes, and touches timestamps of related rows in a separate table.

Arguments:

main_table: name of table that is being watched for changes
touch_table: name of table that needs to be touched
column: name of timestamp column to be touched
expr: hash or array that represents the columns that define the relationship
opts: options hash

License¶ ↑

This library is released under the MIT License. See the MIT-LICENSE file for details.

Author¶ ↑

Jeremy Evans <code@jeremyevans.net>

Vydia/sequel_postgresql_triggers

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