How to use existing DB with IdentityServer4

I started with the IdentityServer4 Quick Start sample 7_JavaScriptClient.

First I created a UserManager class that could connect to my data source and validate a user by user name and password and return that user object.

public class UserManager
{
    private SecurityContext _context;

    public UserManager(SecurityContext context)
    {
        _context = context;
    }

    public  Task<User> Find(string username, string password)
    {
         ...Logic to query your custom user table(s)...
    }

    public Task<List<Claim>> GetClaimsAsync(User user)
    {
        var claims = new List<Claim>();

        //custom database call here to where you store your claims.
        var myClaims = ...Your Database call here...
        var claimGroupName = "SomeCustomName";

        if (security != null && security.Count() > 0)
        {
            foreach (var claim in security)
            {
                //Add the value from the field Security_Id from the database to the claim group "SomeCustomName".
                claims.Add(new Claim(claimGroupName , claim.SECURITY_ID));
            }
        }

        return Task.FromResult(claims);


    }
 }

User Object

public class User
{
    public string FIRST_NAME { get; set; }
    public string LAST_NAME { get; set; }
    public string EMAIL { get; set; }

    ...Other attributes/properties...
}

Second I used the UserManager Object from the AccountController in the sample.

private readonly UserManager _userManager;

public AccountController(
        IIdentityServerInteractionService interaction,
        IClientStore clientStore,
        IHttpContextAccessor httpContextAccessor,
        UserManager userManager
        )
{
    _userManager = userManager;
 }

Third in the AccountController.Login() HTTP Post method I called the UserManager.Find(username, password) to return a user.

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Login(LoginInputModel model)
{
     // validate username/password
     var user = await _userManager.Find(model.Username, model.Password);

     //sign the user in with a subject[user_id] and name[web_id]
     await HttpContext.Authentication.SignInAsync(user.USER_ID, user.WEB_ID, props);
}

Fourth I implemented the IProfileService. [I used this article as a resource.]

public class ProfileService : IProfileService
{
     UserManager _myUserManager;
     private readonly ILogger<ProfileService> _logger;

     public ProfileService(ILogger<ProfileService> logger)
    {
        _logger = logger;
        _myUserManager = new UserManager(new SecurityContext());
    }

    //Called by IdentityServer Middleware.
     public async Task GetProfileDataAsync(ProfileDataRequestContext context)
    {
        var sub = context.Subject.FindFirst("sub")?.Value;
        if (sub != null)
        {
            var user = await _myUserManager.FindByNameAsync(sub);

            //Call custom function to get the claims from the custom database.
            var cp = await   getClaims(user);

            var claims = cp.Claims;

            ...Optionaly remove any claims that don't need to be sent...

            context.IssuedClaims = claims.ToList();
        }
     }

     //Called by IdentityServer Middleware.
     public async Task IsActiveAsync(IsActiveContext context)
    {
        var sub = context.Subject.GetSubjectId();
        var user = await _myUserManager.FindByNameAsync(sub);
        context.IsActive = user != null;
        return;
    }

    //Custom function to get claims from database via the UserManager.GetClaimsAsync() method.
    private async Task<ClaimsPrincipal> getClaims(User user)
    {
       var id = new ClaimsIdentity();
       //set any standard claims
       id.AddClaim(new Claim(JwtClaimTypes.PreferredUserName, user.USER_ID));
       //get custom claims from database or other source.
       id.AddClaims(await _myUserManager.GetClaimsAsync(user));

       return new ClaimsPrincipal(id);
    }
}

Lastly in the Startup.cs setup any Objects for Dependency Injection.

public void ConfigureServices(IServiceCollection services)
{
   builder.Services.AddTransient<IProfileService, ProfileService>();

   //This is the DbContext to our Database where the users and their claims are stored.
   services.AddTransient<SecurityContext>();
   services.AddTransient<UserManager>();

 }

Here is a similar question and anser as well.

Note that they also reference the IResourceOwnerPasswordValidator interface and implementation which is for authenticating OAuth 2.0 resource owner password credential grant (aka password). Used with GrantTypes.ResourceOwnerPasswordAndClientCredentials or GrantTypes.ResourceOwnerPassword.