SQL Server & SSDT : Error modeling database during Publish after adding CREATE AGGREGATE / EXTERNAL NAME I am using SSDT to create a database containing CLR user defined aggregates.
The aggregates are intended to sit in different schema. That is not possible with CLR objects inside SSDT, without using a post-deployment script to move the objects after deployment.
What I am attempting to do is:

Create the CLR aggregates in a single schema (the &quot;CLR&quot; schema) using the &quot;Default schema&quot; option
Use a CREATE AGGREGATE wrapper in the desired schema to call the CLR aggregate

For example, I have a CLR aggregate like:
[Serializable]
[Microsoft.SqlServer.Server.SqlUserDefinedAggregate(Format.UserDefined, MaxByteSize = -1)]
public struct StatsEntropy : IBinarySerialize
{
 ... etc
}

This get compiled and becomes CLR.StatsEntropy in the database. The database is called AnalysisFunctions, so it has an assembly also called AnalysisFunctions. This is no problem.
I then create a wrapper in the Stats schema like so:
CREATE AGGREGATE [Stats].[Entropy]
(
    @Values FLOAT
)
RETURNS FLOAT
EXTERNAL NAME AnalysisFunctions.StatsEntropy

This works without error when I build via debug. I have my debug connection set to a named instance of SQL Server. If I press F5, it compiles and deploys without any problem. The code is now usable on the server, including the wrapper.
But, if I attempt to publish to a server using Publish, it does not work. Instead it fails with:

Errors occurred while modeling the target database. Deployment cannot continue.

There are no further error messages either in the Data Tools Operations panel or error list. ssdttrace shows no useful information:

This only happens when the database already exists on the server. If I drop the database, it can deploy successfully. I can deploy successfully via debug even if the database already exists, it is only via Publish that the problem occurs. I did not start getting this problem until I added the aggregate wrappers.
Previously I was also getting errors indicating that the assembly AnalysisFunctions had an unresolved reference due to the use of EXTERNAL NAME - again working via debug, not via publish. But now that error has mysteriously vanished.
I am using Visual Studio 2017 15.9.21 and SSDT 15.1.62002.01090, deploying to SQL Server 2019.
Does anyone have any idea what this error might be, or how I can debug it?
I can't reproduce this problem using the same version of SSDT as you.  I do have a slightly newer Visual Studio (15.9.24), but I'd be surprised if that matters.  Here's my (very minimal) setup, please let me know if I've misunderstood some part of your scenario:
SSDT Project

clr.sql
CREATE SCHEMA [clr]

Stats.sql
CREATE SCHEMA [Stats]

StatsEntropy.cs
using System;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;

[Serializable]
[SqlUserDefinedAggregate(Format.Native)]
public struct StatsEntropy
{
    public void Init()
    {
        // Put your code here
    }

    public void Accumulate(SqlString Value)
    {
        // Put your code here
    }

    public void Merge(StatsEntropy Group)
    {
        // Put your code here
    }

    public SqlString Terminate()
    {
        // Put your code here
        return new SqlString(string.Empty);
    }

    // This is a place-holder member field
    public int _var1;
}

Default Schema

Normally that failure during preview generation indicates that there are objects in your target database that don't exist in the SSDT model (dacpac).  That's discussed on Stack Overflow here: SSDT Publish errors on Creating Publish Preview
You can use the &quot;Schema Compare&quot; feature to check and see if that's the case for you.
If there are no conflicting objects in the target, then it would be helpful if you could edit your question to include your publish settings (the &quot;.publish.xml&quot; file, or at least screenshots of the &quot;Advanced&quot; window when publishing).  It could be some odd combination of options causing the problem.