Introduce inheritance

Breaking

• Breaking: remove() parent from child side in reverse ForeignKey relation now requires passing a relation name,
  as the same model can be registered multiple times and ormar needs to know from which relation on the parent you want to remove the child.
• Breaking: applying limit and offset with select_related is by default applied only on the main table before the join -> meaning that not the total
  number of rows is limited but just number of main models (first one in the query, the one used to construct it). You can still limit all rows from db response with limit_raw_sql=True flag on either limit or offset (or both)
• Breaking: issuing first() now fetches the first row ordered by the primary key asc (so first one inserted (can be different for non number primary keys - i.e. alphabetical order of string))
• Breaking: issuing get() without any filters now fetches the first row ordered by the primary key desc (so should be last one inserted (can be different for non number primary keys - i.e. alphabetical order of string))
• Breaking (internal): sqlalchemy columns kept at Meta.columns are no longer bind to table, so you cannot get the column straight from there

Features

• Introduce inheritance. For now two types of inheritance are possible:
  • Mixins - don't subclass ormar.Model, just define fields that are later used on different models (like created_date and updated_date on each child model), only actual models create tables, but those fields from mixins are added
  • Concrete table inheritance - means that parent is marked as abstract=True in Meta class and each child has its own table with columns from the parent and own child columns, kind of similar to Mixins but parent also is a (an abstract) Model
  • To read more check the docs on models -> inheritance section.
• QuerySet first() can be used with prefetch_related

Fixes

• Fix minor bug in order_by for primary model order bys
• Fix in prefetch_query for multiple related_names for the same model.
• Fix using same related_name on different models leading to the same related Model overwriting each other, now ModelDefinitionError is raised and you need to change the name.
• Fix order_by overwriting conditions when multiple joins to the same table applied.

Docs

• Split and cleanup in docs:
  • Divide models section into subsections
  • Divide relations section into subsections
  • Divide fields section into subsections
• Add model inheritance section
• Add API (BETA) documentation