[Issue] get_column_values now only accepts a relation

[clrcrl]

Steps to reproduce

1. Install dbt-utils v0.2.1
2. Call get_column_values with a string table reference

{{ dbt_utils.get_column_values(
    table="raw_jaffle_shop.customers",
    column='first_name',
    max_records=50
) }}

Expected results

• Macro compiles correctly (this was the behaviour <0.2.1)

Actual results

Running with dbt=0.14.0
Found 10 models, 28 tests, 0 snapshots, 0 analyses, 249 macros, 0 operations, 0 seed files, 3 sources

16:53:00 | Concurrency: 1 threads (target='dev')
16:53:00 |
Encountered an error:
Runtime Error
  Compilation Error in macro get_column_values (macros/sql/get_column_values.sql)
    'str object' has no attribute 'database'

Commentary

Prior to v0.2.1, you could pass get_column_values a table like this my_schema.my_table.

However #152 now means you need to pass the macro a relation instead. All of these are valid:

{{ dbt_utils.get_column_values(
    table=ref('stg_jaffle_shop__customers'),
    column='first_name',
    max_records=50
) }}

{{ dbt_utils.get_column_values(
    table=source('raw_jaffle_shop', 'customers'),
    column='first_name',
    max_records=50
) }}

{%- set source_relation = adapter.get_relation(
      database=target.database,
      schema='raw_jaffle_shop',
      identifier="customers") -%}

{{ dbt_utils.get_column_values(
    table=source_relation,
    column='first_name',
    max_records=50
) }}

I'm totally 100% behind this change, but there needs to be some cleaning up here to communicate it.

• The README.md still says shows an example of passing a hardcoded table reference in the pivot example
• Techhhhhnically we should replace the table argument name with relation, but that's likely going to break things (on the fence about whether we should fix it)
• There's also a little bit of redundant code in get_column_values which would be good to clean up:

    {%- set target_relation = adapter.get_relation(database=table.database,
                                          schema=table.schema,
                                         identifier=table.identifier) -%}

can be replaced with

    {%- set target_relation = table -%}

[clrcrl]

@emilieschario – I'm happy to fix this, but also happy for you to take this one :)

[emilieschario]

Thanks so much for taking this @clrcrl! I was traveling this weekend!