Description

Change prejoin-processing to only produce one left join for each unique combination of relation, columns and operator.

Also add new syntax for prejoins, allowing extraction of multiple columns from the same prejoin-source:

  prejoined_columns:
      - extract_columns: 
           - id
           - number
        aliases:
           - businessid
           - businessnumber
        ref_model: 'business_raw'
        this_column_name: 'ContractId'
        ref_column_name: 'ContractId'

In this example we join the source model with business_raw and extract id and number and assign an alias for each column.
Assigning an alias is optional, but if done, the number of extract_columns has to match the amount of aliases.
For each prejoin-target a new list, containing at least a dictionary for extract_columns and values for ref_model(or source_name & source_table) and the column names, needs to be defined.

The old syntax is still valid and can be used if desired.

Fixes #287

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation or included information that needs updates

ToDO: Update Docs to include the new syntax and additionally include information about aliases