diff --git a/spec/granite/connection_management_spec.cr b/spec/granite/connection_management_spec.cr
index 995f73e1..06a6a5d9 100644
--- a/spec/granite/connection_management_spec.cr
+++ b/spec/granite/connection_management_spec.cr
@@ -5,10 +5,31 @@ describe "Granite::Base track time since last write" do
     ReplicatedChat.connection_switch_wait_period = 250
     ReplicatedChat.new(content: "hello world!").save!
     sleep 500.milliseconds
+    Fiber.current.granite_adapters.try(&.clear)
     current_url = ReplicatedChat.adapter.url
     reader_connection = Granite::Connections["#{ENV["CURRENT_ADAPTER"]}_with_replica"]
     raise "Reader connection cannot be nil" if reader_connection.nil?
     reader_url = reader_connection[:reader].url
     current_url.should eq reader_url
   end
+
+  it "should support isolated concurrent adapter routing across fibers" do
+    ReplicatedChat.switch_to_writer_adapter
+    # In the main fiber, ReplicatedChat.adapter should be the writer
+    ReplicatedChat.adapter.should eq ReplicatedChat.writer_adapter
+
+    ch = Channel(Nil).new
+
+    spawn do
+      # Switch to reader in this concurrent fiber
+      ReplicatedChat.switch_to_reader_adapter
+      ReplicatedChat.adapter.should eq ReplicatedChat.reader_adapter
+      ch.send(nil)
+    end
+
+    ch.receive
+
+    # Back in the main fiber, it should still be the writer adapter
+    ReplicatedChat.adapter.should eq ReplicatedChat.writer_adapter
+  end
 end
diff --git a/spec/granite/validation_helpers/inequality_spec.cr b/spec/granite/validation_helpers/inequality_spec.cr
index 5b845d4b..c68452ab 100644
--- a/spec/granite/validation_helpers/inequality_spec.cr
+++ b/spec/granite/validation_helpers/inequality_spec.cr
@@ -20,10 +20,8 @@ describe Granite::ValidationHelpers do
       less_than_test.errors[3].message.should eq "float_32_lte must be less than or equal to 100.25"
 
       less_than_test_nil = Validators::LessThanTest.new
-
-      expect_raises(Exception, "Nil assertion failed") do
-        less_than_test_nil.save
-      end
+      less_than_test_nil.save
+      less_than_test_nil.errors.size.should eq 4
     end
   end
 
@@ -46,10 +44,8 @@ describe Granite::ValidationHelpers do
       greater_than_test.errors[3].message.should eq "float_32_lte must be greater than or equal to 100.25"
 
       greater_than_test = Validators::GreaterThanTest.new
-
-      expect_raises(Exception, "Nil assertion failed") do
-        greater_than_test.save
-      end
+      greater_than_test.save
+      greater_than_test.errors.size.should eq 4
     end
   end
 end
diff --git a/spec/granite/validation_helpers/lenght_spec.cr b/spec/granite/validation_helpers/lenght_spec.cr
index 75db1f8e..628cd53e 100644
--- a/spec/granite/validation_helpers/lenght_spec.cr
+++ b/spec/granite/validation_helpers/lenght_spec.cr
@@ -14,5 +14,17 @@ describe Granite::ValidationHelpers do
       length_test.errors[0].message.should eq "title is too short. It must be at least 5"
       length_test.errors[1].message.should eq "description is too long. It must be at most 25"
     end
+
+    it "should handle nil values correctly for length validations" do
+      length_test = Validators::LengthTest.new
+      length_test.title = nil
+      length_test.description = nil
+      length_test.save
+
+      # title being nil fails min length validation -> 1 error
+      # description being nil passes max length validation -> 0 errors
+      length_test.errors.size.should eq 1
+      length_test.errors[0].message.should eq "title is too short. It must be at least 5"
+    end
   end
 end
diff --git a/src/granite/columns.cr b/src/granite/columns.cr
index 1dd65fff..79c9a50c 100644
--- a/src/granite/columns.cr
+++ b/src/granite/columns.cr
@@ -89,16 +89,20 @@ module Granite::Columns
       end
 
       def {{decl.var.id}}! : {{not_nilable_type}}
-        raise NilAssertionError.new {{@type.name.stringify}} + "#" + {{decl.var.stringify}} + " cannot be nil" if @{{decl.var}}.nil?
-        @{{decl.var}}.not_nil!
+        val = @{{decl.var}}
+        raise NilAssertionError.new {{@type.name.stringify}} + "#" + {{decl.var.stringify}} + " cannot be nil" if val.nil?
+        val
       end
+
     {% else %}
       def {{decl.var.id}}=(@{{decl.var.id}} : {{type.id}}); end
 
       def {{decl.var.id}} : {{type.id}}
-        raise NilAssertionError.new {{@type.name.stringify}} + "#" + {{decl.var.stringify}} + " cannot be nil" if @{{decl.var}}.nil?
-        @{{decl.var}}.not_nil!
+        val = @{{decl.var}}
+        raise NilAssertionError.new {{@type.name.stringify}} + "#" + {{decl.var.stringify}} + " cannot be nil" if val.nil?
+        val
       end
+
     {% end %}
   end
 
diff --git a/src/granite/connection_management.cr b/src/granite/connection_management.cr
index bd654ff0..6a450611 100644
--- a/src/granite/connection_management.cr
+++ b/src/granite/connection_management.cr
@@ -1,3 +1,9 @@
+require "atomic"
+
+class Fiber
+  property granite_adapters : Hash(String, Granite::Adapter::Base)?
+end
+
 module Granite::ConnectionManagement
   macro included
     # Default value for the time a model waits before using a reader
@@ -5,36 +11,60 @@ module Granite::ConnectionManagement
     # all models use this value. Change it
     # to change it in all Granite::Base models.
     class_property connection_switch_wait_period : Int32 = Granite::Connections.connection_switch_wait_period
-    @@last_write_time = Time.instant
 
-    class_property current_adapter : Granite::Adapter::Base?
+    # Note: @@reader_adapter and @@writer_adapter remain unsynchronized class-level globals.
+    # This is safe assuming connection configurations are initialized at application startup only.
     class_property reader_adapter : Granite::Adapter::Base?
     class_property writer_adapter : Granite::Adapter::Base?
 
-    def self.last_write_time
-      @@last_write_time
-    end
+    {% if compare_versions(Crystal::VERSION, "1.20.0") >= 0 %}
+      @@epoch = Time.instant
+      @@last_write_time = Atomic(Int64).new(0_i64)
 
-    # This is done this way because callbacks don't work on class mthods
-    def self.update_last_write_time
-      @@last_write_time = Time.instant
-    end
+      def self.last_write_time : Time::Instant
+        @@epoch + @@last_write_time.get.milliseconds
+      end
+
+      # This is done this way because callbacks don't work on class mthods
+      def self.update_last_write_time
+        @@last_write_time.set((Time.instant - @@epoch).total_milliseconds.to_i64)
+      end
+
+      def self.time_since_last_write : Time::Span
+        Time.instant - last_write_time
+      end
+    {% else %}
+      @@epoch = Time.monotonic
+      @@last_write_time = Atomic(Int64).new(0_i64)
+
+      def self.last_write_time : Time::Span
+        @@epoch + @@last_write_time.get.milliseconds
+      end
+
+      # This is done this way because callbacks don't work on class mthods
+      def self.update_last_write_time
+        @@last_write_time.set((Time.monotonic - @@epoch).total_milliseconds.to_i64)
+      end
+
+      def self.time_since_last_write : Time::Span
+        Time.monotonic - last_write_time
+      end
+    {% end %}
 
     def update_last_write_time
       self.class.update_last_write_time
     end
 
-    def self.time_since_last_write
-      Time.instant - @@last_write_time
-    end
-
     def time_since_last_write
       self.class.time_since_last_write
     end
 
     def self.switch_to_reader_adapter
       if time_since_last_write > @@connection_switch_wait_period.milliseconds
-        @@current_adapter = @@reader_adapter
+        fiber_adapters = Fiber.current.granite_adapters ||= {} of String => Granite::Adapter::Base
+        if reader = @@reader_adapter
+          fiber_adapters[self.name] = reader
+        end
       end
     end
 
@@ -43,7 +73,10 @@ module Granite::ConnectionManagement
     end
 
     def self.switch_to_writer_adapter
-      @@current_adapter = @@writer_adapter
+      fiber_adapters = Fiber.current.granite_adapters ||= {} of String => Granite::Adapter::Base
+      if writer = @@writer_adapter
+        fiber_adapters[self.name] = writer
+      end
     end
 
     def switch_to_writer_adapter
@@ -53,12 +86,9 @@ module Granite::ConnectionManagement
     def self.schedule_adapter_switch
       return if @@writer_adapter == @@reader_adapter
 
-      spawn do
-        sleep @@connection_switch_wait_period.milliseconds
-        switch_to_reader_adapter
-      end
-
-      Fiber.yield
+      # In M:N multithreading, spawning a fiber to mutate global state or Fiber local state
+      # is no longer safe or deterministic. We rely on the dynamic check in `adapter` method
+      # and the Fiber-local scope.
     end
 
     def schedule_adapter_switch
@@ -66,10 +96,26 @@ module Granite::ConnectionManagement
     end
 
     def self.adapter
-      begin
-        @@current_adapter.not_nil!
-      rescue NilAssertionError
-        Granite::Connections.registered_connections.first?.not_nil![:writer]
+      fiber_adapters = Fiber.current.granite_adapters
+
+      if fiber_adapters && (adapter = fiber_adapters[self.name]?)
+        return adapter
+      end
+
+      if time_since_last_write > @@connection_switch_wait_period.milliseconds
+        if reader = @@reader_adapter
+          return reader
+        end
+      else
+        if writer = @@writer_adapter
+          return writer
+        end
+      end
+
+      if first_conn = Granite::Connections.registered_connections.first?
+        first_conn[:writer]
+      else
+        raise "No registered connections found"
       end
     end
   end
@@ -82,10 +128,12 @@ module Granite::ConnectionManagement
 
     #{Granite::Connections.registered_connections.map{ |conn| "#{conn[:writer].name}"}.join(", ")}"
 
-    raise error_message if Granite::Connections[{{name}}].nil?
-
-    self.writer_adapter = Granite::Connections[{{name}}].not_nil![:writer]
-    self.reader_adapter = Granite::Connections[{{name}}].not_nil![:reader]
-    self.current_adapter = @@writer_adapter
+    if conn = Granite::Connections[{{name}}]
+      self.writer_adapter = conn[:writer]
+      self.reader_adapter = conn[:reader]
+    else
+      raise error_message
+    end
+    # self.current_adapter = @@writer_adapter
   end
 end
diff --git a/src/granite/validation_helpers/inequality.cr b/src/granite/validation_helpers/inequality.cr
index 08ab181a..45f10d5c 100644
--- a/src/granite/validation_helpers/inequality.cr
+++ b/src/granite/validation_helpers/inequality.cr
@@ -1,9 +1,9 @@
 module Granite::ValidationHelpers
   macro validate_greater_than(field, amount, or_equal_to = false)
-    validate {{field}}, "#{{{field}}} must be greater than#{{{or_equal_to}} ? " or equal to" : ""} #{{{amount}}}", Proc(self, Bool).new { |model| (model.{{field.id}}.not_nil! {% if or_equal_to %} >= {% else %} > {% end %} {{amount.id}}) }
+    validate {{field}}, "#{{{field}}} must be greater than#{{{or_equal_to}} ? " or equal to" : ""} #{{{amount}}}", Proc(self, Bool).new { |model| ((val = model.{{field.id}}) ? (val {% if or_equal_to %} >= {% else %} > {% end %} {{amount.id}}) : false) }
   end
 
   macro validate_less_than(field, amount, or_equal_to = false)
-    validate {{field}}, "#{{{field}}} must be less than#{{{or_equal_to}} ? " or equal to" : ""} #{{{amount}}}", Proc(self, Bool).new { |model| (model.{{field.id}}.not_nil! {% if or_equal_to %} <= {% else %} < {% end %} {{amount.id}}) }
+    validate {{field}}, "#{{{field}}} must be less than#{{{or_equal_to}} ? " or equal to" : ""} #{{{amount}}}", Proc(self, Bool).new { |model| ((val = model.{{field.id}}) ? (val {% if or_equal_to %} <= {% else %} < {% end %} {{amount.id}}) : false) }
   end
 end
diff --git a/src/granite/validation_helpers/length.cr b/src/granite/validation_helpers/length.cr
index fac4eae8..5acee325 100644
--- a/src/granite/validation_helpers/length.cr
+++ b/src/granite/validation_helpers/length.cr
@@ -1,9 +1,9 @@
 module Granite::ValidationHelpers
   macro validate_min_length(field, length)
-    validate {{field}}, "#{{{field}}} is too short. It must be at least #{{{length}}}", Proc(self, Bool).new { |model| (model.{{field.id}}.not_nil!.size >= {{length.id}}) }
+    validate {{field}}, "#{{{field}}} is too short. It must be at least #{{{length}}}", Proc(self, Bool).new { |model| ((val = model.{{field.id}}) ? val.size >= {{length.id}} : false) }
   end
 
   macro validate_max_length(field, length)
-    validate {{field}}, "#{{{field}}} is too long. It must be at most #{{{length}}}", Proc(self, Bool).new { |model| (model.{{field.id}}.not_nil!.size <= {{length.id}}) }
+    validate {{field}}, "#{{{field}}} is too long. It must be at most #{{{length}}}", Proc(self, Bool).new { |model| ((val = model.{{field.id}}) ? val.size <= {{length.id}} : true) }
   end
 end