Tag Archives: sqlalchemy

Flask CRUD with sqlalchemy and jinja2 contextfilters

Quick disclaimer, the Flask CRUD thing is not public domain yet and is very volatile.

The project is here
And the outline for the crud thing is in this commit https://github.com/devdave/wfmastery/commit/e249895ddc53c0696f59d3def5718e76855af5b9


First is how the crud is currently constructed

class Equipment(CrudAPI):
    def populate(self):
        self.record_cls = db.Equipment
        self.identity = "equipment"
        self.template_form = "equipment_form.j2.html"
        self.template_list = "equipment_list.j2.html"
        self._listColumn("name", magic_field="magic-string")
        self._listColumn("pretty_name", magic_field="magic-string")
        self._addRelationship("category", "name", magic_field="magic-filter")
        self._addRelationship("subcategory", "name", magic_field="magic-filter")

both vars “template_form” and “template_list” are going to be preset once I am certain that the templates can stand on their own with the context vars provided. The “magic-” params and their use are very much magic (eg really toxic) and would recommend ignoring them.

From there the CrudAPI takes over. Skipping ahead to how this relates to context filters. I had this tag mess here in the template

-{%-      for column_name in origin.list_columns -%}
 -{%-          if column_name in origin.magic_columns -%}
 -        {{ cell("", column_name|title, classes=origin.magic_columns[column_name]) -}}
 -{%-          else -%}
 -        {{ cell("", column_name|title) -}}
 -{%          endif %}
 -{%-      endfor %}

and was really not happy with it. So I dived into Flask and Jinja2’s documentation and code to figure out if I could apply Python code inline.

The answer is yes via jinja2’s contextfilters which are not exposed to Flask but can still be used.

def render_header(context, column_name, value="", **kwargs):
    result = ""
    if column_name in context['origin'].magic_columns:
        result = context['cell'](value, column_name.capitalize(), classes=context['origin'].magic_columns[column_name])
        result = context['cell'](value, column_name.capitalize())
    return result

The trick to going from filter to contextfilter is just applying `my_func.contextfilter = True` outside of your functions scope. From there you have access to almost everything (if not everything). The var “origin” is the CrudAPI’s instance passed to the template.

This has opened a lot more opportunities to do clean up. Taking

{% macro data_attributes(data_map, prefix="data-") -%}
    {%- for name, value in data_map.items() -%}
    {{" "}}{{prefix}}{{name}}="{{value}}"
    {%- endfor -%}
{%- endmacro %}
{% macro cell(name, value, classes=None, data_attrs={}) %}
        <span class="{{- ["cell",classes]|join(" ") if classes else "cell" -}}"{{data_attributes(data_attrs)}}>
      {{- caller() if caller else value -}}</span>
  {%- endmacro -%}

and condensing it down to

{% macro cell(name, value, classes=None, data_attrs={}) %}
        <span class="{{- ["cell",classes]|join(" ") if classes else "cell" -}}" {{data_attrs|dict2attrs("data")|safe}}>
    {{- caller() if caller else value -}}</span>
{%- endmacro -%}

via a simple non-context filter

def dict_to_attributes(attributes, prefix=None):
    results = []
    name2dash = lambda *x: "-".join(x)
    format_str = "%s-{}=\"{}\"" % prefix if prefix else "{}=\"{}\""
    for key, value in attributes.items():
        results.append(format_str.format(key, value))
    #TODO disable autoescape
    return " ".join(results)

Just note that at the moment output is still managed by Jinja’s autoescape and I’d rather not shut that off so calls MUST be suffixed with “|safe” as used above.

As for the Crud API, I feel like that is coming along nicely.