Erlang Web App 2017 04 20

Day 3

Overview

Today's goal is to implement the tag entity and complete the remaining CRUD operations for the initial version of the project.

Person entity

• Update a person's information
• Retrieve all people
• Delete a person (both soft / hard delete)
• Optionally associate a person with a tag upon creation (via many-many associaton table)
• Update a person's tag list (via many-many associaton table)

Tag entity

• Create / Retrieve tags
• Retrieve all tags
• Tags are immutable and can't be deleted (for now)

Tag table

I added the following record and code to create a table consisting only of tag names:

-record(volmgr_tags, {id :: atom()}).

VolmgrTagsOpts = [
            {attributes, record_info(fields, volmgr_tags)},
            {disc_copies, Nodes},
            {type, set}
          ],
{atomic, ok} = mnesia:create_table(volmgr_tags, VolmgrTagsOpts),

However, I got the following error when running tests:

{badmatch,{aborted,{bad_type,volmgr_tags,{attributes,[id]}}}}

Looks like the "table must at least have one extra attribute in addition to the key".

Once I added the active attribute, the test passed.

Writing Multiple Entities

For a test, I needed to write multiple tags to the database, but mnesia:write only works on single records. Ideally, every write would be contained in the same transaction. I needed a recursive anonymous function with multiple clauses, but ran into a compilation issue concerning the variable bound to the function. Some googling revealed that OTP 17 supports "anonymous but named functions" which is what I used:

create_tags(Tags) ->
    Records = [#volmgr_tags{id=Tag, active=true} || Tag <- Tags],
    Writer = fun W([]) ->
                 ok;
             W([Record|T]) ->
                 mnesia:write(Record),
                 W(T)
             end,
    mnesia:activity(transaction, Writer, [Records], mnesia).