[Wilt Chamberlain] Support ref and source in Operations

[drewbanin]

Issue

Issue description

Operations do not currently support ref or source method calls.

An exception is raised when ref is used:

2019-06-05 16:47:27,534 (MainThread): Encountered an error while running operation: Compilation Error in macro say_hi (macros/my-op.sql)
  argument of type 'NoneType' is not iterable
2019-06-05 16:47:27,534 (MainThread): 
Traceback (most recent call last):
  File "/Users/drew/fishtown/dbt/core/dbt/clients/jinja.py", line 104, in call
    return macro(*args, **kwargs)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/runtime.py", line 575, in __call__
    return self._invoke(arguments, autoescape)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/asyncsupport.py", line 110, in _invoke
    return original_invoke(self, arguments, autoescape)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/runtime.py", line 579, in _invoke
    rv = self._func(*arguments)
  File "dbt-a67bac59d05187e871611b73", line 41, in macro
    to_string(environment.call(context, (undefined(name='ref') if l_2_ref is missing else l_2_ref), 'idk')),
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/sandbox.py", line 438, in call
    return __context.call(__obj, *args, **kwargs)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/runtime.py", line 262, in call
    return __obj(*args, **kwargs)
  File "/Users/drew/fishtown/dbt/core/dbt/context/runtime.py", line 44, in do_ref
    if target_model_id not in model.get('depends_on', {}).get('nodes'):
TypeError: argument of type 'NoneType' is not iterable

An exception is also raised when source() is used:

2019-06-05 16:48:52,545 (MainThread): Encountered an uncaught exception while running operation: 'ParsedMacro' object has no attribute 'sources'
2019-06-05 16:48:52,545 (MainThread): 
Traceback (most recent call last):
  File "/Users/drew/fishtown/dbt/core/dbt/task/run_operation.py", line 44, in run
    result = self._run_unsafe()
  File "/Users/drew/fishtown/dbt/core/dbt/task/run_operation.py", line 37, in _run_unsafe
    manifest=manifest
  File "/Users/drew/fishtown/dbt/core/dbt/adapters/base/impl.py", line 894, in execute_macro
    result = macro_function(**kwargs)
  File "/Users/drew/fishtown/dbt/core/dbt/clients/jinja.py", line 104, in call
    return macro(*args, **kwargs)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/runtime.py", line 575, in __call__
    return self._invoke(arguments, autoescape)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/asyncsupport.py", line 110, in _invoke
    return original_invoke(self, arguments, autoescape)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/runtime.py", line 579, in _invoke
    rv = self._func(*arguments)
  File "dbt-d96473c053fb049fa5d01868", line 41, in macro
    to_string(environment.call(context, (undefined(name='source') if l_2_source is missing else l_2_source), 'idk', 'id')),
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/sandbox.py", line 438, in call
    return __context.call(__obj, *args, **kwargs)
  File "/Users/drew/fishtown/dbt/env/lib/python3.7/site-packages/jinja2/runtime.py", line 262, in call
    return __obj(*args, **kwargs)
  File "/Users/drew/fishtown/dbt/core/dbt/context/runtime.py", line 81, in do_source
    model.sources.append([source_name, table_name])
  File "/Users/drew/fishtown/dbt/core/dbt/api/object.py", line 125, in __getattr__
    ).format(type(self).__name__, name))
AttributeError: 'ParsedMacro' object has no attribute 'sources'

Results

dbt raised an exception, whereas I expected dbt to interpolate model/source relation representations.

System information

The output of dbt --version:

dev/wilt-chamberlain (ddb1785698fc920fb681a240376ae8cf3c27c78)

The operating system you're running on: osx

The python version you're using (probably the output of python --version) 3.7

Steps to reproduce

Create a model and a source:
models/my_source.yml

version: 2
sources:
  - name: my_source
    tables:
      - name: my_table

models/my_model.sql

select 1 as id

Create and run an operation

{% macro my_model_op() %}
{{ ref('my_model') }}
{% endmacro %}

{% macro my_source_op() %}
{{ source('my_source', 'my_table') }}
{% endmacro %}

$ dbt run-operation --macro my_model_op
$ dbt run-operation --macro my_source_op

[drewbanin]

I am interested in supporting string interpolation of ref and source function calls -- it is less important that these functions actually build edges -- these Operations will not show up in the docs, and we do not need to invoke Operations that depend on a ref/source, for instance.

[drewbanin]

@beckjake can you have a think about how feasible this is?