Sitecore Customize SOLR Managed Schema

CustomManagedSchema.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <contentSearch>
      <customSolrManagedSchema>
        <commands applyToIndex="my_custom_index">

          <add-or-replace-field-type>
            <name>ngram</name>
            <class>solr.TextField</class>
            <positionIncrementGap>100</positionIncrementGap>
            <indexAnalyzer>
              <tokenizer>
                <class>solr.StandardTokenizerFactory</class>
              </tokenizer>
              <filters>
                <class>solr.LowerCaseFilterFactory</class>
              </filters>
              <filters>
                <class>solr.ASCIIFoldingFilterFactory</class>
                <preserveOriginal>false</preserveOriginal>
              </filters>
              <filters>
                <class>solr.NGramFilterFactory</class>
                <minGramSize>2</minGramSize>
                <minGramSize>35</minGramSize>
              </filters>
            </indexAnalyzer>
            <queryAnalyzer>
              <type>query</type>
              <tokenizer>
                <class>solr.StandardTokenizerFactory</class>
              </tokenizer>
              <filters>
                <class>solr.LowerCaseFilterFactory</class>
              </filters>
              <filters>
                <class>solr.ASCIIFoldingFilterFactory</class>
                <preserveOriginal>false</preserveOriginal>
              </filters>
            </queryAnalyzer>
          </add-field-type>
          
          <add-dynamic-field>
            <name>*_ngram</name>
            <type>ngram</type>
            <indexed>true</indexed>
            <stored>true</stored>
            <termVectors>true</termVectors>
            <termPositions>true</termPositions>
            <termOffsets>true</termOffsets>
          </add-dynamic-field>
          <add-dynamic-field>
            <name>*_ngramm</name>
            <type>ngram</type>
            <indexed>true</indexed>
            <stored>true</stored>
            <termVectors>true</termVectors>
            <termPositions>true</termPositions>
            <termOffsets>true</termOffsets>
            <multiValued>true</multiValued>
          </add-dynamic-field>
        </commands>
      </customSolrManagedSchema>
    </contentSearch>
    <pipelines>
      <contentSearch.PopulateSolrSchema>
        <!-- Processor for additional SOLR schema -->
        <processor type="MyProject.Pipelines.ContentSearch.PopulateSolrSchema.CustomSchemaPopulator, MyProject"
                   patch:after="processor[@type='Sitecore.ContentSearch.SolrProvider.Pipelines.PopulateSolrSchema.PopulateFields, Sitecore.ContentSearch.SolrProvider']"/>
      </contentSearch.PopulateSolrSchema>
    </pipelines>
  </sitecore>
</configuration>

CustomSchemaPopulator.cs

using System.Collections.Generic;
using System.Linq;
using System.Xml;
using System.Xml.Linq;
using Sitecore.Configuration;
using Sitecore.ContentSearch.SolrProvider.Pipelines.PopulateSolrSchema;
using Sitecore.Diagnostics;
using Sitecore.Xml;
using SolrNet.Schema;

namespace MyProject.Pipelines.ContentSearch.PopulateSolrSchema
{
    public class CustomSchemaPopulator : PopulateFields
    {
        protected string IndexName;

        public override void Process(PopulateManagedSchemaArgs args)
        {
            IndexName = args.Index.Name;
            base.Process(args);
        }

        protected override ISchemaPopulateHelper GetHelper(SolrNet.Schema.SolrSchema schema)
        {
            Assert.ArgumentNotNull(schema, "schema");
            return new CustomSchemaPopulatorHelper(schema, IndexName);
        }
    }

    public class CustomSchemaPopulatorHelper : ISchemaPopulateHelper
    {
        private readonly SolrSchema _solrSchema;
        private readonly string _indexName;

        public CustomSchemaPopulatorHelper(SolrSchema solrSchema, string indexName)
        {
            _solrSchema = solrSchema;
            _indexName = indexName;
        }

        public IEnumerable<XElement> GetAllFields()
        {
            var elements = new List<XElement>();

            foreach (XmlNode commands in Factory.GetConfigNodes("contentSearch/customSolrManagedSchema/commands"))
            {
                if (IsApplicable(commands, _indexName))
                {
                    elements.AddRange(commands.ChildNodes.Cast<XmlNode>().Select(n => ParseCommand(n.OuterXml, _solrSchema)));
                }
            }

            return elements;
        }

        private XElement ParseCommand(string xml, SolrSchema schema)
        {
            var element = XElement.Parse(xml);
            
            // set "add" or "replace" depending if field type already exists in schema
            if (element.Name == "add-or-replace-field-type")
            {
                var fieldTypeName = element.Elements().FirstOrDefault(e => e.Name == "name");
                if (fieldTypeName != null && !string.IsNullOrEmpty(fieldTypeName.Value))
                {
                    if (schema.SolrFieldTypes.Any(f => f.Name == fieldTypeName.Value))
                    {
                        element = new XElement("replace-field-type", element.Attributes(), element.Elements());
                    }
                    else
                    {
                        element = new XElement("add-field-type", element.Attributes(), element.Elements());
                    }
                }
            }

            return element;
        }

        private bool IsApplicable(XmlNode commands, string indexName)
        {
            var indexes = XmlUtil.GetAttribute("applyToIndex", commands);
            if (string.IsNullOrEmpty(indexes))
            {
                return false;
            }

            return indexes.Split('|').Any(i => i.ToLower().Equals(indexName.ToLower()));
        }
    }
}

Hi @D4rkii
Thanks for the feedback. It seems that in 9.1 the values from GetAllFieldTypes() are sent to Solr first and then the values from GetAllFields(). Maybe this is to prevent trying to add fields which do not have a type yet. In my example this doesn't happen as the add-or-replace-field-type command is processed first.
To do it right, I would run the add-field-type and replace-field-type commands in the GetAllFieldTypes() method and all the others in the GetAllFields() method. I haven't had the chance to test this on 9.1 though.