#postgres

In our project, we used gem Recorder. It saves changes in jsonb format.

Once we need to rename a column in model Contact, but we didn't want to lose logs.

I introduce you template of migration for this case.

 class RenameOldColumnToNewColumn < ActiveRecord::Migration[5.2] def up rename_column :contacts, :old_column, :new_column execute <<~SQL UPDATE recorder_revisions SET data = jsonb_set(data #- '{changes,old_column}', '{changes,new_column}', data#>'{changes,old_column}') WHERE data#>'{changes}'?'old_column'; SQL end def down rename_column :contacts, :new_column, :old_column execute <<~SQL UPDATE recorder_revisions SET data = jsonb_set(data #- '{changes,new_column}', '{changes,old_column}', data#>'{changes,new_column}') WHERE data#>'{changes}'?'new_column'; SQL end end 

You can simple replace contacts, new_column and old_column to your table name and columns names.

Say we have a table with a column of an array type. At some point, we want to be able to select records with a specific value(s) which the array column may have.

Here are three ways to do different kinds of searches.

1) Use ANY operator when searching with one value:

 SELECT * FROM mytable WHERE 'first_type' = ANY(types_column); 

2) Go with the "contains" operator (“@>”) when you look for a specific set of values (the order of values doesn’t matter):

 SELECT * FROM mytable WHERE types_column @> '{"first_type", "second_type"}'; 

The values “first_type” and “second_type" must be in the types_column column for a record to be selected.

3) Whenever you need to search any values that a column may have - use the "overlap" operator (“&&”)

 SELECT * FROM mytable WHERE types_column && '{"first_type", "second_type"}'; 

One of the values “first_type” or “second_type" must be in the types_column column for a record to be selected.

ActsAsTaggableOn is a swiss army knife solution if you need to add tags to your ActiveRecord model.

Just by adding one gem to your Gemfile and acts_as_taggable to the model you get everything you need: adding tags, searching for a model by tag, getting top tags, etc. However, sometimes you don't need all these.

In our project, we used acts_as_taggable to store tags for Note model. Then we displayed a list of notes on several pages with assigned tags and had autocompleted input for tags on Note form. Everything worked well, but since we use PostgreSQL, I decided to store tags as an array in Note model.

First of all, I added tags Array<String> column to Note, after this migrated actsastaggable tags to notes table with migration.

 class MigrateNoteTags < ActiveRecord::Migration[5.2] def change execute <<-SQL UPDATE notes SET tags = grouped_taggings.tags_array FROM ( SELECT taggings.taggable_id, ARRAY_AGG ( tags.NAME ) tags_array FROM taggings LEFT JOIN tags ON taggings.tag_id = tags.ID WHERE taggable_type = 'Note' GROUP BY taggings.taggable_id ) AS grouped_taggings WHERE notes.ID = grouped_taggings.taggable_id SQL end end 

To have backward compatibility, I added Note#tag_list method:

 def tag_list tags.join(', ') end 

The last thing is to add the ability to search for tags. Since there about 500k records in the Notes table, I decided to create an SQL view:

 CREATE OR REPLACE VIEW note_tags AS SELECT UNNEST ( tags ) AS name, COUNT ( * ) AS taggings_count FROM notes GROUP BY name 

That's it! It takes from 100ms to 150ms to search for tags in this view, which is fine for me.

If you have more significant data sets, then the best would be to create tags table and add triggers to notes table that will update tags on INSERT/UPDATE/DELETE.

PostgreSQL provides a inet and cidr datatypes for storing net addresses and proceed operations with them.

Host address and it's subnet can be stored with inet, while cidr can contain only network address:

 select inet '192.168.0.1/24'; inet ---------------- 192.168.0.1/24 

 select cidr '192.168.0.0/24'; -- valid cidr cidr ---------------- 192.168.0.0/24 

 select cidr '192.168.0.1/24'; -- invalid: cidr must not be a host address ERROR: invalid cidr value: "192.168.0.1/24" LINE 1: select cidr '192.168.0.1/24'; ^ DETAIL: Value has bits set to right of mask. 

In case there's no number after slash in cidr address the netmask is to equal 32:

 select cidr('127.0.0.1'); cidr -------------- 127.0.0.1/32 

The value above represents a subnet address, while the same value passed to inet represents a host:

 select inet('127.0.0.1'); inet ----------- 127.0.0.1 

Checking inclusion or equality can be performed with >>= and <<= operators:

 select inet '192.168.0.1/24' >>= inet '192.168.0.0'; -- returns true select cidr '192.168.0.0/24' >>= inet '192.168.0.0/12'; -- returns false select cidr '192.168.0.0' >>= cidr '192.168.0.0'; -- returns true 

And getting a netmask by a net address can be performed with netmask:

 select netmask(inet('192.168.0.0/24')); -- returns 255.255.255.0 select netmask(cidr('127.0.0.1')); -- returns 255.255.255.255 

You have to insert some data into your PostgreSQL table that has line break. As a result, the cell will look like this:

 First line Second line 

To use "escape sequences" in a string literal you need to use an "extended" constant. You can specify the escape character by prefixing the letter E:

 UPDATE posts SET body = E'First Line\nSecond line.' WHERE id = 2701;