Adds section about multiple insertion/update to api/writing docs [#405]

docs/api/writing.md

@@ -71,6 +71,93 @@ returns something like
 [ { "id": 1 }, { "id": 2 } ]
 ```
 
+### Multiple Tables Insertion or Update
+
+The cleanest way to insert or update data into multiple tables using only one POST/PATCH request
+is to create a view that will join all target tables and present a single endpoint.
+In our example let's assume one users table and one companies table.
+In this case, we want a signup endpoint to create the first user within a company.
+And for this endpoint we want to insert with one request both user and company.
+
+```SQL
+CREATE TABLE companies (
+    id serial primary key,
+    name text unique
+);
+
+CREATE TABLE users (
+    id serial primary key,
+    name text not null,
+    pass text,
+    company_id integer not null references companies
+);
+```
+
+Having both tables created we create a view that joins them to be used
+as a ```/signup``` endpoint.
+
+```SQL
+CREATE VIEW signup AS
+    SELECT
+        c.name AS company_name,
+        u.name AS user_name,
+        u.pass
+    FROM
+        public.users u
+        JOIN public.companies c ON c.id = u.company_id;
+
+```
+
+After the signup view creation, we can issue ```GET``` requests to read data
+from users and companies, but any atempt to ```POST``` or ```PATCH``` data will fail.
+PostgreSQL won't allow any data change on views that have a ```JOIN``` 
+clause in their ```FROM``` without a proper ```INSTEAD OF``` trigger.
+So in the example bellow we create a trigger to allow insertion of data in the signup view.
+The trigger is a simple PL/pgSQL function that first inserts into the companies table and
+uses the newly create company_id to create its first user.
+
+
+```SQL
+CREATE FUNCTION signup()
+RETURNS trigger
+LANGUAGE plpgsql
+AS $$
+DECLARE
+  vcompany_id int;
+BEGIN
+  INSERT INTO companies (name) VALUES (new.company_name) RETURNING id INTO vcompany_id;
+  INSERT INTO users (name, pass, company_id) VALUES (new.user_name, new.pass, vcompany_id);
+RETURN new;
+END;
+$$;
+
+CREATE TRIGGER signup
+INSTEAD OF INSERT ON signup
+FOR EACH ROW
+EXECUTE PROCEDURE signup();
+```
+
+After the trigger creation we can issue a normal ```POST``` request to our signup endpoint:
+
+```HTTP
+POST /signup
+{ "company_name": "foo", "user_name": "bar" }
+```
+
+For an endpoint such as signup its usually not desirable to have a ```PATCH``` route for updates,
+and we will skip this example for the sake of brevity. But it would be implemented in a very
+similar way to our ```POST``` example.
+
+<div class="admonition note">
+    <p class="admonition-title">Design Consideration</p>
+
+    <p>It's advisable to create a separate trigger for <code>UPDATE</code> and <code>INSERT</code>
+    avoiding conditionals that decide which is the trigger current operation.
+    This makes it easier to change code for (or even disable) one operation without intefering with others while
+    improving readability.
+    </p>
+</div>
+
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