introduce new API for Model().

```ruby
step Model(Song, find_by: :short_id, params_key: :slug)
```

This allows to use arbitrary finders and using a key in `params` different to
the column name.

lib/trailblazer/macro/model.rb

@@ -1,8 +1,29 @@
 module Trailblazer
   module Macro
 
-    def self.Model(model_class = nil, action = :new, find_by_key = :id, id: 'model.build', not_found_terminus: false)
-      task = Activity::Circuit::TaskAdapter.for_step(Model.new)
+      # TODO: deprecate find_by_key in favor of `find_by: :id`
+    def self.Model(model_class = nil, action = :new, find_by_key = :id, id: "model.build", not_found_terminus: false, params_key: find_by_key, **options)
+      # {find_by: :slug}
+      if options.any?
+        raise "unknown options #{options}" if options.size > 1
+
+        id_from = ->(ctx, params:, **) { params[params_key] } # TODO: We can hand in other behavior here, Yogi!
+
+        extract_id = Macro.task_adapter_for_decider(id_from, variable_name: :id)
+
+        action, find_by_key = options.to_a[0]
+
+        task = Activity::Railway() do
+          step task: extract_id, id: :extract_id
+          step Model.method(:produce)
+        end
+
+        options = Activity::Railway.Subprocess(task)
+      else # old style, deprecate
+        task = Activity::Circuit::TaskAdapter.for_step(Model.new)
+
+        options = {task: task, id: id}
+      end
 
       injections = {
         Activity::Railway.Inject() => [:params], # pass-through {:params} if it's in ctx.
@@ -12,10 +33,12 @@ def self.Model(model_class = nil, action = :new, find_by_key = :id, id: 'model.b
           :"model.class"          => ->(*) { model_class },
           :"model.action"         => ->(*) { action },
           :"model.find_by_key"    => ->(*) { find_by_key },
+          :"model.id_from"        => ->(*) { id_from }, # TODO: test me.
         }
       }
 
-      options = {task: task, id: id}.merge(injections)
+      options = options.merge(injections)
+
 
       options = options.merge(Activity::Railway.Output(:failure) => Activity::Railway.End(:not_found)) if not_found_terminus
 
@@ -30,11 +53,19 @@ def call(ctx, params: {}, **)
         ctx[:model] = model
       end
 
+      def self.produce(ctx, id:, **)
+        model_class   = ctx[:"model.class"]
+        find_by_key   = ctx[:"model.find_by_key"]
+
+        ctx[:model] = model_class.find_by(find_by_key.to_sym => id)
+      end
+
       class Builder
         def call(ctx, params)
           action        = ctx[:"model.action"]
           model_class   = ctx[:"model.class"]
           find_by_key   = ctx[:"model.find_by_key"]
+          id_from       = ctx[:"model.id_from"]
           action        = :pass_through unless %i[new find_by].include?(action)
 
           send("#{action}!", model_class, params, ctx[:"model.action"], find_by_key)

test/docs/model_test.rb

@@ -118,6 +118,44 @@ class Update < Trailblazer::Activity::Railway
   #~ctx_to_result end
 end
 
+class DocsModelFindBy___Test < Minitest::Spec
+  Song = Struct.new(:id, :short_id) do
+    def self.find_by(short_id:)
+      return if short_id.nil?
+      new(1, short_id)
+    end
+  end
+
+  #:find_by_column
+  module Song::Activity
+    class Update < Trailblazer::Activity::Railway
+      step Model(Song, find_by: :short_id, params_key: :slug)
+      step :validate
+      step :save
+      #~meths
+      include T.def_steps(:validate, :save)
+      #~meths end
+    end
+  end
+  #:find_by_column end
+
+  #~ctx_to_result
+  it do
+    #:find_by_column-invoke
+    signal, (ctx, _) = Trailblazer::Activity.(Song::Activity::Update, params: {slug: "1f396"}, seq: [])
+    ctx[:model] #=> #<struct Song id=2, slug="1f396">
+    #:find_by_column-invoke end
+
+    assert_equal ctx[:model].inspect, %{#<struct #{Song} id=1, short_id="1f396">}
+  end
+
+  it do
+    signal, (ctx, _) = Trailblazer::Activity.(Song::Activity::Update, params: {slug: nil}, seq: [])
+    assert_equal ctx[:model].inspect, %{nil}
+  end
+  #~ctx_to_result end
+end
+
 class DocsModelAccessorTest < Minitest::Spec
   Song = Class.new(DocsModelTest::Song)