Moving ORM Mapping towards the database

Posted on July 6, 2017

Storing data in a relational database has it’s roots in the late sixties of the past century. The core idea has survived the last decades. About 2009 the term NoSQL appeared.

As for now PostgreSQL is the most advanced relational database in the world. With version 9 you can store non atomic data in a JSON column. Document based NoSQL databases like MongoDb are storing there data in so called collections. These collections are similar to PostgreSQL JSON columns.

With PostgreSQL you are able to use the best of both worlds.

Some tables

Before entering the JSON world, let’s look at a simple example. I use the pgcrypto extension for generating id columns for the tables.

CREATE EXTENSION IF NOT EXISTS pgcrypto;

For fast prototyping, you can use an own schema for the example.

DROP SCHEMA IF EXISTS test CASCADE; CREATE SCHEMA test; SET search_path TO test,public;

If you like to store personal data you start with a person

CREATE TABLE person ( id BUD NOT NULL PRIMARY KEY DEFAULT gen_random_uuid(), first_name VARCHAR(512), last_name VARCHAR(512), birth_date DATE, notes VARCHAR(4096), website VARCHAR(256) );

With an address table,

CREATE TABLE address ( id UUID NOT NULL PRIMARY KEY DEFAULT gen_random_uuid(), street VARCHAR(512), house_number VARCHAR(128), postal_code VARCHAR(10), city VARCHAR(512) );

you can store several addresses for a person.

CREATE TYPE address_type AS ENUM ( 'private', 'delivery', 'invoice', 'work' ); CREATE TABLE person_to_address( id_person UUID NOT NULL REFERENCES person (id), id_address UUID NOT NULL REFERENCES address(id), is_primary_address boolean NOT NULL DEFAULT false, address_type address_type NOT NULL DEFAULT 'private', PRIMARY KEY (id_person, id_address) );

A simple table for storing emails can look like

CREATE TABLE email ( id UUID NOT NULL PRIMARY KEY DEFAULT gen_random_uuid(), eMail_address VARCHAR(256) ); CREATE TYPE communication_type AS ENUM ( 'work', 'private', 'organization' ); CREATE TABLE person_to_email ( id_person UUID NOT NULL REFERENCES person (id), id_email UUID NOT NULL REFERENCES email (id), communication_type communication_type NOT NULL DEFAULT 'private', is_primary_email_address BOOLEAN NOT NULL DEFAULT false, PRIMARY KEY (id_person, id_email) );

Similar to an email you can store phone data like

CREATE TYPE communication_network AS ENUM ( 'landline', 'cellular_network' ); CREATE TABLE phone ( id UUID NOT NULL PRIMARY KEY DEFAULT gen_random_uuid(), phone_number VARCHAR(128) NOT NULL, communication_network communication_network NOT NULL DEFAULT 'landline' ); CREATE TABLE person_to_phone ( id_person UUID NOT NULL REFERENCES person (id), id_phone UUID NOT NULL REFERENCES phone (id), communication_type communication_type NOT NULL DEFAULT 'private', is_primary_phone_number BOOLEAN NOT NULL DEFAULT false, PRIMARY KEY (id_person, id_phone) );

If you like to need meta data for every table, like last update date or create date , you can do this with a simple trigger function.

CREATE FUNCTION metadata_trigger() RETURNS TRIGGER AS $$ BEGIN NEW.updated_at := now(); RETURN NEW; END $$ LANGUAGE plpgsql;

In the next step, you add to every table a created_at and a updated_at column. After this the metadata_trigger trigger function has to be added.

CREATE FUNCTION add_metadata_to_every_table() RETURNS VOID AS $$ DECLARE row record; BEGIN FOR row IN SELECT tablename FROM pg_tables WHERE schemaname = 'test' LOOP EXECUTE 'ALTER TABLE ' || row.tablename || ' ADD COLUMN created_at timestamp NOT NULL DEFAULT NOW();'; EXECUTE 'ALTER TABLE ' || row.tablename || ' ADD COLUMN updated_at timestamp NOT NULL DEFAULT NOW();'; EXECUTE 'CREATE TRIGGER ' || row.tablename || '_trigger BEFORE UPDATE ON ' || row.tablename || ' FOR EACH ROW EXECUTE PROCEDURE metadata_trigger();'; END LOOP; END $$ LANGUAGE plpgsql;

No stunts so far.

Prior to the NoSQL movement, you would probably create a view for a person .

CREATE VIEW person_view AS SELECT first_name, last_name, street, house_number, postal_code, city, email_address, phone_number FROM PERSON p JOIN person_to_address p2a ON p.id = p2a.id_person JOIN address a on p2a.id_address = a.id JOIN person_to_email p2e on p2e.id_person = p.id JOIN email e on e.id = p2e.id_email JOIN person_to_phone p2p on p2p.id_person = p.id JOIN phone ph on ph.id = p2p.id_phone;

You get a tabular result with many redundant data here. The next layer will take this raw data and transform it into objects.

$ psql -U postgres -c "select * from test.person_view" first_name | last_name | street | house_number | postal_code | city | address_type | email_address | phone_number --------------+-----------+------------+---------------+-------------+----------+--------------+------------------+---------------- Jan Frederik | Hake | No Street | 3-4 | 54321 | Dortmund | work | jan_hake@fake.de | +4923111223344 Jan Frederik | Hake | Fakestreet | 123 | 12345 | Dortmund | private | jan_hake@fake.de | +4923111223344 Jan Frederik | Hake | No Street | 3-4 | 54321 | Dortmund | work | jan_hake@fake.de | +4915199887766 Jan Frederik | Hake | Fakestreet | 123 | 12345 | Dortmund | private | jan_hake@fake.de | +4915199887766 (4 rows)

It would be nice, if the database it self could provide these objects. At this point, the JSON columns come into the game.

JSON column

In this example the person table is our root relation. We add the json column in this table.

ALTER TABLE person ADD COLUMN json_view JSONB;

In the first step we create a function, that fills this column.

CREATE FUNCTION update_json_view_person(person_id UUID) RETURNS VOID AS $$ DECLARE person_raw JSONB; BEGIN SELECT row_to_json(p) FROM (SELECT id, first_name, last_name, birth_date, notes, website FROM person WHERE id = person_id) p INTO person_raw; UPDATE person SET json_view = person_raw WHERE id = person_id; END $$ LANGUAGE plpgsql;

A sample output for json_view can look like

$ psql -U postgres -c "select json_view from test.person" json_view ------------------------------------------------------------------------------------------------------------------------------------------------------- {"id": "e881de40-596d-47f1-801c-77bf32829bfa", "notes": null, "website": null, "last_name": "Hake", "birth_date": null, "first_name": "Jan Frederik"} (1 row)

There are many json functions available for Postgres. The row_to_json function will create a json object for every result row. In this case it is just one row.

When you want to add the address data you can use the array_agg aggregate function to create an array from a result,

SELECT array_to_json(array_agg(addresses)) FROM (SELECT a.id, street, house_number, postal_code, city, p2a.address_type FROM address a JOIN person_to_address p2a ON a.id = p2a.id_address WHERE p2a.id_person = person_id) addresses INTO person_addresses;

where person_to_addresses is a local JSONB variable. The array_to_json function creates a json array, which can be added to the person_raw with json_build_object .

person_raw := person_raw || jsonb_build_object('addresses', person_addresses);

You can do this similar for email and phone .

The complete function looks like

CREATE FUNCTION update_json_view_person(person_id UUID) RETURNS VOID AS $$ DECLARE person_raw JSONB; person_addresses JSONB; person_email_addresses JSONB; person_phone_numbers JSONB; BEGIN SELECT row_to_json(p) FROM (SELECT id, first_name, last_name, birth_date, notes, website FROM person WHERE id = person_id LIMIT 1) p INTO person_raw; SELECT array_to_json(array_agg(addresses)) FROM (SELECT a.id, street, house_number, postal_code, city, p2a.address_type FROM address a JOIN person_to_address p2a ON a.id = p2a.id_address WHERE p2a.id_person = person_id) addresses INTO person_addresses; SELECT array_to_json(array_agg(email_addresses)) FROM (SELECT e.id, email_address, is_primary_email_address, communication_type FROM email e JOIN person_to_email p2e on e.id = p2e.id_email WHERE p2e.id_person = person_id) email_addresses INTO person_email_addresses; SELECT array_to_json(array_agg(phone_numbers)) FROM (SELECT p.id, phone_number, communication_type, communication_network, is_primary_phone_number FROM phone p JOIN person_to_phone p2p on p.id = p2p.id_phone WHERE p2p.id_person = person_id) phone_numbers INTO person_phone_numbers; person_raw := person_raw || jsonb_build_object('addresses', person_addresses) || jsonb_build_object('email_addresses', person_email_addresses) || jsonb_build_object('phone_numbers', person_phone_numbers); UPDATE person SET json_view = person_raw WHERE id = person_id; END $$ LANGUAGE plpgsql;

Now the person looks more or less complete

$ psql -U postgres -c "select json_view from test.person" | cat json_view ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ {"id": "5824be75-b444-4ac7-8d59-0763e6a6a9b3", "notes": null, "website": null, "addresses": [{"id": "41a93a1b-fd31-4f05-8a63-8921a926223c", "city": "Dortmund", "street": "Fakestreet", "postal_code": "12345", "address_type": "private", "house_number": "123"}, {"id": "4a2558c9-13b5-49a8-89b8-52022575040b", "city": "Dortmund", "street": "No Street", "postal_code": "54321", "address_type": "work", "house_number": "3-4"}], "last_name": "Hake", "birth_date": null, "first_name": "Jan Frederik", "phone_numbers": [{"id": "86941ea5-fe53-4251-bdfc-abafca40b4ab", "phone_number": "+4923111223344", "communication_type": "private", "communication_network": "landline", "is_primary_phone_number": true}, {"id": "96b8ebd3-f514-4fd7-997c-136e4a6eb270", "phone_number": "+4915199887766", "communication_type": "private", "communication_network": "cellular_network", "is_primary_phone_number": false}], "email_addresses": [{"id": "9fc2ea91-cf68-4624-a903-381d765be25c", "email_address": "jan_hake@fake.de", "communication_type": "private", "is_primary_email_address": false}]} (1 row)

With a little bit formatting you get.

{ "id": "5824be75-b444-4ac7-8d59-0763e6a6a9b3", "notes": null, "website": null, "addresses": [{ "id": "41a93a1b-fd31-4f05-8a63-8921a926223c", "city": "Dortmund", "street": "Fakestreet", "postal_code": "12345", "address_type": "private", "house_number": "123" }, { "id": "4a2558c9-13b5-49a8-89b8-52022575040b", "city": "Dortmund", "street": "No Street", "postal_code": "54321", "address_type": "work", "house_number": "3-4" }], "last_name": "Hake", "birth_date": null, "first_name": "Jan Frederik", "phone_numbers": [{ "id": "86941ea5-fe53-4251-bdfc-abafca40b4ab", "phone_number": "+4923111223344", "communication_type": "private", "communication_network": "landline", "is_primary_phone_number": true }, { "id": "96b8ebd3-f514-4fd7-997c-136e4a6eb270", "phone_number": "+4915199887766", "communication_type": "private", "communication_network": "cellular_network", "is_primary_phone_number": false }], "email_addresses": [{ "id": "9fc2ea91-cf68-4624-a903-381d765be25c", "email_address": "jan_hake@fake.de", "communication_type": "private", "is_primary_email_address": false }] }

Every time the update_json_view_person function is called, the json_view column is updated with the current relational data.

In the next part, I take a look at some other use cases.