feature: allow setting discriminator value manually

This commit allows using an instance of a base class in a
Single Table Inheritance scenario and setting the discriminator value
manually.

Related to: typeorm#9033

src/metadata/EntityMetadata.ts

@@ -861,8 +861,13 @@ export class EntityMetadata {
             this.inheritancePattern === "STI" &&
             this.childEntityMetadatas.length > 0
         ) {
+            // There could be a column on the base class that can manually be set to override the type.
+            let manuallySetDiscriminatorValue: unknown
+            if (this.discriminatorColumn) {
+                manuallySetDiscriminatorValue = value[this.discriminatorColumn.propertyName]
+            }
             return this.childEntityMetadatas.find(
-                (meta) => value.constructor === meta.target,
+                (meta) => manuallySetDiscriminatorValue === meta.discriminatorValue || value.constructor === meta.target,
             ) || this
         }
         return this