whoosh schema in haystack and django

I am trying to integrate a whoosh searcher into a django project. I saw that you can do that using haystack but I am realizing I can't (dont know yet) how to add my custom whoosh index into the searcher. My schema has ID, KEYWORD and TEXT but they are all text in reality. I used these schemes because it suits my search needs for each of the documents. How do I use this schema in Haystack.

PS: A solution without Haystack is ok too.

Here is my whoosh schema/writer/searcher

import pandas as pd
from whoosh.index import create_in
from whoosh.fields import *
from whoosh.qparser import QueryParser
from whoosh.query import *

def nan2none(x):
    y = None if pd.isna(x) else x
    return(y)

df = pd.read_csv("df.csv", index_col=[0])

schema = Schema(a = ID(stored=True),
                b = KEYWORD(lowercase=True),
                c = TEXT,
                d = KEYWORD(lowercase=True))

ix = create_in("indexdir", schema)
writer = ix.writer()

for index, row in df.iterrows():
    writer.add_document(a = index,
                        b = nan2none(row['b']),
                        c = nan2none(row['c']),
                        d = nan2none(row['d']))
writer.commit()

search_term = "hobbit"
with ix.searcher() as searcher:
    a_query = QueryParser("a", ix.schema).parse(search_term)
    b_query = QueryParser("b", ix.schema).parse(search_term)
    c_query = QueryParser("b", ix.schema).parse(search_term)
    d_var_query = QueryParser("d", ix.schema, termclass=Variations).parse(search_term)
    d_fuzz_query = QueryParser("d", ix.schema, termclass=FuzzyTerm).parse(search_term)

    query = Or([a_query, b_query, c_query, d_var_query, d_fuzz_query])
    results = searcher.search(query, limit=None)
    print(results)
    for res in results:
        print(res)

But in my django model all the documents I am adding above are CharField as follows:

class ModelLetters(modes.model):
    a = models.CharField(max_length=50)
    b = models.CharField(max_length=100)
    c = models.CharField(max_length=100)
    d = models.CharField(max_length=250)

Whereas my haystack index is as follows (all CharField too):

from haystack import indexes
from appmanager.model.model_letters import ModelLetters


class LettersIndex(indexes.SearchIndex, indexes.Indexable):
    text = indexes.CharField(document=True, use_template=True)

    a = indexes.CharField(model_attr="a")
    b = indexes.CharField(model_attr="b")
    c = indexes.CharField(model_attr="c")
    d = indexes.CharField(model_attr="d")

    class Meta:
      model = ModelLetters
      fields = ["a", "b", "c", "d"]


    def get_model(self):
        return ModelLetters

    def index_queryset(self, using=None):
        """Used when the entire index for model is updated."""
        return self.get_model().objects.all()