Add doc for adding custom column convention

articles/custom-column-convention.md

@@ -0,0 +1,53 @@
+---
+uid: custom-column-convention
+title: Creating a Custom ColumnConvention
+---
+
+# Creating a Custom Column Convention
+
+In certain instances, it may be required to add your own customized column conventions. This is
+useful for supporting custom column formats that aren't covered by FluentMigrator
+
+First, create your custom column convention by implementing `IColumnsConvention`. In this example, a column convention is added that supoprts a JSON column like so:
+
+```csharp
+class MyCustomColumnConvention : IColumnsConvention {
+  private readonly string _databaseType;
+
+  public MyCustomColumnConvention(IProcessorAccessor processorAccessor) {
+    _databaseType = processorAccessor.Processor.DatabaseType.ToLowerInvariant();
+  }
+
+  public IColumnsExpression Apply(IColumnsExpression expression) {
+    var jsonColumns = expression.Columns
+      .Where(x => string.Equals("json", x.CustomType, StringComparison.OrdinalIgnoreCase))
+      .ToList();
+    if (jsonColumns.Count == 0) {
+      return expression;
+    }
+
+    if (_databaseType == "sqlserver2016") {
+      foreach (var column in jsonColumns) {
+        column.CustomType = null;
+        column.Type = DbType.String;
+        column.Size = int.MaxValue;
+      }
+    }
+
+    return expression;
+  }
+}
+```
+
+Option 1, useful if you already depend on behavior of `IConventionSetAccessor`, Add your convention to the existing IConventionSet:
+
+```csharp
+var conventionSet = services.GetRequiredService<IConventionSet>();
+var processorAccessor = services.GetRequiredService<IProcessorAccessor>();
+conventionSet.ColumnConventions.Add(new MyCustomColumnConvention(processorAccessor));
+```
+
+Option 2, doesn't rely on assembly scanning and requires:
+
+1. A custom `IConventionSet` as scoped service, which should be derived from `DefaultConventionSet` and have only one public constructor,
+2. The custom `IColumnsConvention` must be added to the `ColumnsConventions` property of `IConventionSet`