Clarify differences between database backends, and clarify third argument in create_table

[darltrash]

I have a moonscript file like the following:

import Model from require "lapis.db.model"
import create_table, types from require "lapis.db.schema"

create_table "invite_codes", {
    { "id", types.serial }

    "PRIMARY KEY (id)"
}

class InviteCodes extends Model
    @primary_key: "id"

return {
    :InviteCodes
}

and i get the following:

SQL: CREATE TABLE "invite_codes" (
  "id" nil,
  PRIMARY KEY (id)
)

now, I have two questions, is this normal? am I misusing this? and, isn't it meant to be CREATE TABLE "invite_codes" IF NOT EXISTS instead?

[darltrash]

Apparently create_tables has a third, undocumented argument which is a table of options, I could clarify that if necessary

[darltrash]

#807 Made a PR to clarify this.